Does that mean a higher developmental level reads bigger quantities?
It depends what you exactly mean by "higher developmental level" and what the "quantities" are. But as you get better with reading yes you can read more of the music at once and react with what you have to do to produce those notes accurate and more immediately.
1. Does/should reading size vary with the grade of a piece?
With "reading size" I think you mean the amount of the score you can notice rather than the actual size of the notes themselves. If you take a very easy piece you should be able to read while playing larger parts at a time and have a lot of time to solve what comes next with time to spare! Even high grade pieces have parts which are quite easy to read and others which may require time to calculate and thus shrinks your reading focus (a position we like to avoid so that our reading experience is easily controlled).
2. Does it vary with score density?
You could have a progression of many thick chords and it could be much easier than a fast single string of notes doing all sorts of acrobatics. If something is very fast tempo this limits the amount you are able to read the most, these fiddly little passages can really tax your attention and make it difficult to see what reacts with them. Usually sight readers will simply slow down to maintain control or use other tools like pausing, neglecting notes etc to deal with it on initial reads to maintain coherent sight reading flow.
3. Can it vary from measure to measure?
It varies from measure to measure based on your past experience with what you are reading and how confident you are with all the fingering/coordination solutions that you have used in the past to solve what you are reading.
4. Could polyrhythms, or thick vertical harmonies limit how much further one could read ahead?
It depends on the individual. If a piece asks for some kind of unfamiliar coordination that is highly repetitive throughout it is not that difficult to read so long the initial calculation of how to do it is appropriately solved and fully mastered. This is how a piece that might seem difficult at first becomes easier to read the more you repeat through it, the difficult parts become solved and then the same solution is applied throughout. This is only if you really understand what you are playing, if you are merely parroting a phrase of music in isolation and not fully understanding how similar ideas are used else where then this process wont work very well.
I don't think I read bass clef as fluently as treble clef. There are pieces/times where both hands are in treble clef. I think I read these in bigger chunks.
Almost all cases I come across who say they read one clef better than the other simply haven't have enough experience reading the other clef. You learned another instrument so that is why you are better at the treble since you have countless hours practicing reading in that clef. Simply read a whole lot of bass line passages for a month and you will see an improvement.
Another example, I don't think I read chords/intervals nearly as fluently as scales. So, maybe smaller chunks here as well. I'm note sure.
Chords should be easier than scales. One random tip for calculating a thick chord, start with the lowest note first, then solve the highest, the centre should then more readily reveal itself based on its interval relationship to these max and min points of the chord. Also chord progression has particular sounds to them which you get very accustomed to which make calculating how chords move much easier.
...unstable coordination (never heard of that, could you elaborate with some examples please?)
When you memorize a piece and your muscular memory is not so precise and has inefficient movements. You could even call it "estimate playing" where you are sort of getting the right sound but it is tarnished with strange movements, strange mannerisms that have crept in through multiple repeats of the poor motions. For example someone who doesn't understand syncopation can estimate how the notes in both hands interact with one another but it is unstable, has little errors spotted throughout, the mind of the player doesn't even follow the sound of the syncopation and it feels like an unsteady solution which is "sort of" correct but not.
Could unstable coordination be a contributing factor to the seemingly "random mistakes"? Maybe they are, in fact, not so random after all?
Well sure, it can even have a level of error which needs to be covered up with the pedal or blurring notes or rushing through the passage, playing one hand louder to cover up the errors of the other etc etc.
There are "very easy pieces" that I just can't play/figure out on my own. I have to watch a YouTube video to see the elusive movements and then go, "Oh, that's how you play that"!
Is it possible that movement issues can cause some of the speed walls?
Very hard to learn these movements on your own, Joe.
Well that is what makes the piano a tricky instrument to master. It is easy to make sound out of the piano but to play it with the most economical fingering and movements, that is where the challenge really lies. The difference between correct and incorrect can often be small. Many simply solve what is correct by the good feeling they are having in their hand but then again piano can trap you because not everything that is intuitive and feels comfortable is correct, sometimes we need to learn new movements which may feel alien at first but become so ingrained they become a natural reaction. I know many early beginners I have taught find it easier to rest their wrist on the wood of the keyboard while playing, I know this is an extreme example but they really feel it's easier. There are many fingering ideas which we may feel is correct but is not necessarily the best solution that we should be attuned to.
...I found for me, that in this piece, reading one beat ahead works fairly well. That size, keeps me fluid and I don't get lost or overwhelmed by information overload. Evidently, a half measure at a time, best fits my developmental stage. Hmm... that means my eyes keep tempo with the metronome. Interesting, never thought about that.
Often when I read pieces which I easily can handle I predominatnly don't need to read ahead in terms of solving what comes next before I get there, this is since I can just read on the fly and solve it quite easily. If I notice something is coming up which requires a little more I will look at it earlier and solve it before I get to it. It is just a natural reaction to notice what is ahead to see if it is the same as what you are doing since in that case there is no need to read ahead with the intention for precalculation and if the changes look quite simple you will feel confident to simply solve it when you get there. If there is something that is coming up which is different then you should be reading ahead and ensure you will be ready to deal with it.
Topic: What Bach Invention #1 did for me