Bigger is a bit of a problem.
It'd require me to redraw the entire sprite sheet, realign the coordinates for each individual limb, reassign the equipment - in short, it's probably not going to happen.
Smaller is possible. Actually, right now you can eat dwarfberries to shrink yourself. Eventually we could have something where you could be younger, so you're permanently a little bit smaller than usual. I don't mind that at all, it's just a bit lower on the priority list. ;P
Gender differences by height - eh. The *best* way to go about it would be to change the statues of the characters entirely - but it'll be a bunch of work (especially with spritesheets and such). Though, males and females looking the same was never really a problem. You could always ask, though since it's the Internets, they can tell you whatever. The only reason the game asks you for your gender is so it knows, whether NPCs should address you as a "he" or a "she". ;P
TLDNR: Smaller - sure, bigger - probably not. Gender specific sizes - I'm not convinced. ;P