Well, you haven't said which program you are using, but it's actually pretty simple.
There is no limit on character sprite size. So all that you'd have to do would be to resize the graphic in an art program. (Bear in mind, of course, that this will make it blurry.)
Anyway, yeah, if you want to have a full-sized dragon in the party, just select that full-sized dragon from the charset. There's absolutely nothing stopping you.