Hold on for a moment. Let's back up a check out this recommendation. I have a 32+ inch draw and shoot both 70 and 72 inch bows. You are not going to find any low, or even moderate cost 27 inch risers. There is no market for them. There are a few companies that make them. Plus there are a few extra long ILF limbs out there. Unless you have a really long draw length, and have been shooting long enough to warrant that type of bow, then 72 inches is probably a reach. Send us your experience, height and draw length. Also, a bit about your coach. Is this someone who teaches at a shop or a someone who coaches higher end archers? It does not take much to get a coaching certification. You can get level one in a day, and level two in a weekend.
I also shoot a 70 inch longbow. I have heard that you can't get longer than that because of the fiberglass strips that are used. I am not sure this is true. I heard it while I was looking for mine. I have a board bow that is about 75 inches long, but then, pulling a board 32 inches is pretty frightening so you want the bow that is as long as possible.