Kathy: Welcome to QB!!
I have an Olfa 45mm rotary cutter, and I buy replacement blades for it from Harbor Freight, a hardware place. They are 45mm carpet cutter replacement blades. I would assume that all 45mm blades will fit 45mm cutters, but you'd have to do your own experiments on that. I don't think that you can put one size of blade in another size of cutter.
For your charity quilts: The first thing you might do is to ask the charity what is most wanted/needed. I'm sure that any quilt you make and donate will be well loved and well used, but you don't want to give them something that they cannot use. From my experience working with homeless families with children, I'd say that the best size quilt is twin size, the next best is queen size, if the object is that the family will take the quilt with them to their new home.