What's a receding nail line, and what defines "stubby" fingers? I think my hands are pudgy and small, but my friends say they're bony and quite large.

A nail line that is receded is when the nail becomes unattached to the skin. If you grow out your nails a little bit, you'll see that the nail that hangs out is lighter in color than the nail that is still attached to the skin. The nail line is that "line" at the edge of where it's still attached.
After playing the piano, and playing the keys is manners that pushes the skin off the nail, it recedes. Nail biters are notorious for having severely receded nail lines. Sometimes, they have very little nail attached... YUCK! And what's worse: they keep on biting even though there isn't any nail that is hanging free to be bitten! YUCK!
When your nail line recedes to a certain point and you are manicured so that little of the nail hangs off the nail line, the finger tips look stubby.
What's worse about a receding nail line is that once it becomes unattached, it won't grow back. But it will grow/attach back as long as the nail has been receded to the point where the nail is still alive. The upper region of the nail is alive but as it grows out, it starts dying. The last third of your nail is dead keratin. (Keratin is what your nail is made of. So are rhinoceros horns.)
Stubby fingers, I define as this: When the nail line has receded past the point where the nail no longer supports the tip of the fingers and when manicured, the flesh is exposed when looking at the fingertips from above.
Many pianists suffer from this stubby fingers syndrome. Daniel Barenboim, that thin white woman with the blondish hair and short stature has some nasty hands - very stubby, that tall black guy (not Andre Watts but he could have it too)... et cetera. My 5 fingers have receded 1mm over the last month as I was learning an etude (Alkan's Op. 35-5, Allegro barbaro) that was about octaves played really fast and loud. Damn octaves! Fun to play, though.
Usually, the tip of the nails get "worn" broad and are no longer round but flatter. Czerny says Beethoven's finger tips have been "worn broad". Heres a site about left hand music but it includes a picture of the cast of Beethoven's hand alongside Chopin's. It's somewhere in the middle of the page. That's what I consider to be broad. Also to note, M.A. Hamelin has broad fingers.
https://hjem.get2net.dk/Brofeldt/Here's a site that gives the anatomy of the nail. It uses different terminoloy. I think my lay terms make more sense to lay people, though.
https://dermatology.about.com/cs/nailanatomy/a/nailanatomy.htm