It depends a lot on how you like to fly.

If you want to fly scale or pattern look at planes in the 7-8 lbs. (most 60 size) range. The 40 size 3D planes are designed to be overpowered so the .91 will be ok in them. I wouldn't put a .91 in a regular (non 3D) 40 size . A 60 size 3D plane may need a little more ummmp than a .91 can provide to do 3D. As stated above if you just want to fly around and not 3D then the .91 will be ok in about any 60 size plane. If you want power to spare then you are going to have to put the .91 in a lighter plane. Again it just depends on how you like to fly.