Most pianists can actually reach further than they think if they can stretch a little more between the thumb and second finger, as well as stretching the pinkie a little further out.
I've seen pianists that thought they could only reach a ninth, but have the potential to reach most tenths, and even in some cases elevenths (though it may not be as functional).
For larger reaches, like Db to F (LH), try lifting the E a little bit with the fleshy part of the thumb and then pressing the F with the tip of the thumb so that the E doesn't sound when reaching that tenth.
I can reach all the major tenths and all the elevenths like F# - B, F - Bb, etc., but I have to clip some notes with the tip of the thumb and sometimes clip just the edge of the key with the pinkie.
Best of luck! You can reach further than you think in most cases, but don't injure yourself by doing too many "stretching" exercises. Too many pianists have hurt their hands and worn out joints by doing too much stretching.