I use the 6.25" reflector with my umbrellas. That reflector is so versatile - because of the wide coverage, I also use it for lighting backgrounds. My umbrellas are 43" Westcott white with removable black backing. The reason I got those is because they usually travel with me, in the studio I mostly use softboxes. If you want a larger umbrella, the 60" Photogenic "Eclipse" white umbrella is well regarded (just ask Robert 
). It's probably the one I'd get if I was looking for a larger umbrella. And the 6.25" reflector won't have trouble covering it with light... 
