With any type of vision problem, bigger is better and you don't need to worry as much about speed as that's why RC engines have throttles. You are looking for a wingspan something in the 65 - 75 inch span (or larger if you wish), with a high wing, with a little dihedral. You want a thick, flat bottom wing which most large high wing trainer-type airplanes have. Some of the popular standby trainers are Tower Hobbies "Tower 40" and if you want even larger, the "Tower 60". Every club in the country that does training probably has had a Sig LT-40. These are great flying airplanes with 70 inch spans and they fly great with a .46 size two stroke and will do nicely with a lowly O.S. 40 LA. I too have vision problems that has made it necessary for me to begin building larger airplanes. I find that a red color scheme with wide yellow bands on the bottom of the wing is very helpful. I have a Sig 4 Star 60 that has 1-1/2 inch wide stripes from the wingtip to the fuselage and that is very helpful for orientation. You won't find it at the AMA Expo but Nitro Planes makes a 60 size high wing "Super Trainer 60" (about a 72 inch span) which I have flown for years with a Magnum 61 2-stroke. This thing has been really put through its paces and it will fly slow, fast (fast is relative of course) and it is also equipped to haul a glider space shuttle style to altitude and then released. The glider pilot brings it back to mother earth. But basically a flat bottom high wing trainer in the .46 - .60 size will fill the bill for you. Let the throttle take care of the fast-slow situation. For visibility and orientation, you will just have to experiment with add-on trim strips. Self adhesive strips are available in the covering section at any hobby shop.