My first response was to say, I think just increasing number of fingers is good. I've fisted my girlfriend that way. Then I realized that my girlfriend's husband is about the same girth as Randy. So perhaps there's something to your idea!
She needs to be really turned on to be elastic enough, but sometimes that isn't enough to take a whole fist. The position of the cervix varies during a woman's cycle and sometimes it's just too low. I prefer to let things flow naturally rather than to have a "goal."
Tuck your thumb into your four fingers and measure the diameter of the widest part of your hand (around the knuckles). I bet you'll find your hand is substantially bigger than Randy. However, once you get past the bridge, it'll be a lot narrower--whereas when you play with Randy, it's all or nothing. So they will definitely be different experiences but I think Randy would be good build-up.