lots of differences
If you are using a Nancy small block and want say 6000RPM stall then a 8" will likely be the best base to set at that stall.
If you have a big inch man motor and want the same stall of 6000 lets say, then a 9.5" to maybe a 10" may be the best to set up for that stall. The larger converter diameter will be easier to lock up and harness a big block in top gear. The smaller diameter will have the same effect on the less powerful mouse motor and free up some power with the smaller diameter (less inertia to rev up). The big motor will rev thru the 8" in top gear and not have enough holding power for the greater power level, generally speaking.
Each manufacturer has there own stator turbine set ups that they interchange to get a specific stall with a given diameter. So the answer will be ultimately the torque converter builders perf for your power level, car weight, gear ratio, etc.........
Does that help?
Bruce