27/27.5 is the same as a size 9,
keep in mind when shell fitting this boot your dealing with a very thin liner (especially once it gets molded), so your going to want to be in a 1 finger shell fit (give or take), If you have a wider fitting foot, you can step up in size a bit, but no more then a two finger fit.  Your going to want just less then a single finger of space width wise in the shell. So if you move your foot to the inside of the boot with the heel all the way to the back, you want to have about a finger of space from the ankle to your toes, between the outside edge of the shell and your foot.   If you just have a couple spots that are too narrow, (ankle points out more, or bunion) these can be fixed with some shell work.
Also since you say you don't live close to a boot fitter I would strongly urge you not to get full tilts, since they will have a very generic fealing fit until that liner gets molded. Also, molding an intuition liner is very different then the standard 'heat molding' of most liners.  If not done correctly it can mess the liner and your fit up pretty well.