<< see, this is how i do it.
(when i first drafted this pattern, i was using bernat "chunky" and a 6.5mm hook almost exclusively. those sizes of yarn and hook are usually my jumping off point when i'm picking new yarn for this project).
as long as you use the same hook and yarn for the hand and the thumb, this pattern should work beautifully, AND be useful for a variety of sizes.
all that said, this pattern is also very easy to modify. unless it's not big enough (or too big) around. then, you'd pretty much be drafting a new pattern anyway, so when that happens, i usually just change yarn. or hook. the path of least resistance.
if you think your hand is longer (or shorter) than mine, add (or subtract) rows between R9-20. same deal for the thumb. actually, the thumb in this pattern is a little on the long side, so i'd recommend leaving out maybe 2 rows of 8sc (between R 4-9).
i'll work on some thumb and ribbing variations for this pattern, and post them later on, too.
sean sheep "maldon"
(this is just what i could find, and the mitten fits my hand, well, like a glove. without fingers. but the stitches are pretty tight. i think the yarn's a bit too big for the hook, but whatever. comment if you're looking for yarn substitution ideas, or if you have questions about this pattern at all. i'm here to help!)
***work in a continuous spiral until instructed otherwise - for the ribbing. also, get yourself a stitch marker. they help a LOT.