I have taught magic for over 20 years. I have never had a student who could not fan due to hand size.
Having said that, it took me FOREVER it seems to learn to Pressure fan. (I was 11, maybe 12ish) Worked on it so hard, and nothing ever came. So I gave up.
About a year later I was going through the book I had used and thought I would try it. Bingo! Instantly it worked.
It's a knack.
My advice: If you want to learn to pressure fan FIRST learn to spring cards from hand to hand. The cards MUST come off the fingertips (shooting forward) not off the thumb (shooting backwards.) You do not need to go for distance. Shooting them an inch or so is fine. What you DO want to practice is smoothness. It should sound like a constant and steady riffle.
Once you have that,you have the basic of a pressure fan. Now do essentially the same thing, while holding the cards at their base in your left hand. Your right hand springs the cards into place (the left hand keeps them from really going anywhere) as it moves up and down in an arc.
If things get weird at the end of the fan, that's normal. It's a bit of a knack figuring out how to move the right hand so it makes a full arc without getting caught up on the left thumb.
When I was teaching these things, I discovered working on the spring first made it so much easier for people to learn the fan, rather than vice versa.
When learning a thumb fan, new, slick cards are essential. When learning a pressure fan, they are sometimes hard to control. When learning, a nice worn in (but not sticky) deck may be best.
Also, if your hands are small, start fan work with bridge size cards.
Brad Henderson