It is my understanding that flat panels work best at the highest resolution. This gives more "real estate" but smaller text. For those with less than perfect vision is this a problem? Would lowering the resolution affect performance?
Can the size of the fonts be changed and if so, would this affect performance?
Your understanding is slightly flawed. Flat panels/LCDs have a native resolution. You can display differently than native with an LCD, but the output will not be as sharp as native. Not at all like a CRT where you have a maximum resolution and you can display anywhere under that without penalty. Technically there is a performance penalty in time the higher color and resolution that you display. However, that mainly applies to slower processors and video cards. Today's processors and video cards are so fast that the time penalty isn't even noticable. I have a 2001FP running at 1600 x 1200 x 32 (native resolution) and my 69 year old father has no problems using it. Some web sites that don't make their fonts scalable he has a problem with, but those are few and far in between.