Years ago when it came to choosing caster size, bigger was better, but this is no longer true. What matters most is the "footprint" of the caster and fork. Smaller and wider casters will increase front stability and also allow more frame clearance if you want a more compact chair. The softroll caster is the most popular because it is about 1.5" wide and doesn't get caught in those troublesome cracks. They come in 3", 4", 5" and 6" diameters but 4" is the most common choice. The larger diameter will limit the front frame angle choices so 5" and 6" diameter is chosen when traversing outdoors.
Invacare Top End offers a short fork that can accommodate multiple sizes but it is best not to plan to change sizes because this will affect the seat heights and the ultimate rollability of your chair.