Hm... I usually go the other way and make patterns smaller
But my thoughts on this...
I think it would help to have a sketch or drawing of Umbreon in the size you want to make so you can always compare your crochet piece and see if you got the size right.
Then, a bigger plushie might need a wire skeleton to stand upright.
Bigger yarn and hook, I'd definitely try that. If that leaves you with not-so-tight stitches, you could use black stuffing (some shredded thrift store garments or so) and it won't be apparent.
Most important if you want to get a certain exact size: gauge! Make trial body parts in different sizes and see which one is the one you need. It's time-cnosuming, but certainly less than finding out you need to redo a whole number of body parts when you have already progressed far.
Some pragmatic thought: Make a small Umbreon first. That way, you'll learn a lot about how it's made AND you have a default present to fall back on if the big one gives you trouble.
Please post your Umbreon when it's done! I'd love to see how it turns out!