I've never been able to take a fist, but, I can comfortably take three fingers and maybe the tip of the thumb when highly aroused. Make no mistake, I can also squeeze hard enough to inflict pain or milk his cock (thank you, Kegels!), but the two qualities are not mutually exclusive.
I can, with relish, open up or tighten for him at will, and that is a skill I am fortunate to have - well, it took a lot of work - but, I am appreciative of my abilities and so are they.
Your lady sounds the same... you're lucky, enjoy it or I'm sure she can find someone else who will. :wink: