Having had sisters and daughters (as well as their mother) to do shopping for and with, I didn't really ever have much of a problem, since, as you have read, most salespeople want the merchandise out the door after your money is in their hand. I have had a couple of smart aleck cashiers remark that I would "look good" in a certain garment, to which I just say "Thank you, I hope so!" after which both of us easily laugh about it, I pay the money and go out the door. (This actually happened one time when I was buying something for my oldest daughter, and she told the cashier "like hell he will, this one is mine!! Neither the cashier or I were able to stop blushing or laughing for quite a while. My daughter was 10 at the time.) I am quite sure that if I ever came through in female mode, they would never identify me as the "guy" who bought something.
For sizing, do look at some of the web sites that have size charts on them, get yourself a good tailor's soft rule, (aka measuring tape) and fit your sizes to theirs to come up with an approximate size. There are many of these web sites, some listed on the resource pages here on Susan's.
My real good suggestion for you though, is find a "->-bleeped-<- Shop" near where you live, and while they may be ARE more expensive than J.C. Penney's, you can shop there and get a lot of help that you can use on your own in the days to come. Have fun!