Terry,
We stopped offering 7/8" deep stretchers as the look of the deeper bars were preferred by our customers. The 1 1/2" and 2" are perfect for just about any size wrap.
We use both sizes regardless of the gallery wrap size. If I were to pick a size where we start using more of the 2" bars it would be at around 20 x 30".
I would skip 7/8" entirely.
I was about to concur, until I read the OP again and realized he doesn't mentioned "wraps", only stretches. If a piece is to be framed after stretching, then 1.5 to 2" bars may indeed be too deep and cause the frame to float out from the wall in a manner that isn't appealing. In this case we normally recommend 3/4 inch bars, and on small prints such as 8x10 I think we even use 1/2" bars. And we don't bother "wrapping" them, just conventional stretch with edge stapling.
If indeed the OP is only considering gallery wraps then I concur Dan ... don't bother with the 7/8. You may want to go to 1.25 for small prints such as 8x10/11x14. (in fact on these small wraps we have decided just to use the system made by IGWrap, and sold through Breathing Color, Liberty Photo, and Hahnemühle ... it's a little more but it's so fast and easy for the little stuff.)