By the sound of it, the 15 suits my needs better. I personally don't like processing too much wood with a knife, that's what a hatchet is for; unless its carving kind,ing or the like
It sounds like you are new to hunting, if you're an old hand, you know all of what follows, in that case, my apologies in advance.
Before you even start to field dress, make sure you are prepared for the job. In modern times, you need to be aware of blood born pathogens, you need to wear protective gloves, if there's even a chance you may have an open cut or wound on your hands (and in the field, isn't that always a possibility?) you should double glove. I prefer surgical gloves, I carry nitrile ones these days (though I miss the old latex ones), I've always got them in my kit anyway. Also carry some string or stout but thin rubber bands, you'll want these to tie off the rectal and genital area. Be very conscious of ticks, they have an annoying habit of acting like heat seeking missiles, as the carcass cools, they'll be looking for their next host and you are the most likely next free meal.
The main points behind field dressing are to prevent meat contamination and spoilage, being careful is a lot more important than being quick.
Don't go after splitting the pelvic bone by hacking at it with your knife. Splitting the pelvic bone on all but the oldest or largest mammals is largely a function of technique, not force, you don't need to chop through it. Usually if you get your knees above the animal's rear hocks and apply outward pressure on them, while applying pressure with your blade (or hatchet blade) not in the exact center of the pelvic bone, but just a little off center, the pelvic bone will separate.
You asked about the choice between a 5 and a 15, I'd say you've made a fine choice in picking the 15, just get it sharp, and if you think there's a chance you'll use it for something that could dull it before you need it for field dressing, carry a way to restore its edge with you.
FWIW, zpstl321 is right, you don't need a large knife for field dressing, too large a blade gets in the way. The blade I usually use is about half as long as the BK15, and once on a dare, I dressed out a Whitetail doe with an exacto knife, it was plenty of blade for the job. I've since met several who use scalpels for the job.
Enjoy your hunt, and enjoy your new knife!
Erik