I don't know where you expect to be hiking, but if you aren't going into mountain country, are hiking in the summer, I don't think denim is a bad idea at all. The new synthetics are lightweight, breathable, and low friction (don't bind when you flex your knees) so they do make a difference, but I've always worn denim and I'm comfortable that way down to about freezing. The other stuff feels like underwear to me. I ran a half marathon in blue jeans a couple of years ago, hottest day of the year, had no problems but did get a lot of strange looks.
For cold weather there's always military surplus. Most of it is wool and rather heavy, but the single layer wool shirts are good enough to make decent hiking jackets. Whether you could find one that fits, well, if you're thin they'd probably hang pretty loose on you. Better stuff than that is Goretex and costs more.
Check out some hunting outlets instead of places like REI. I usually look at REI stuff and wind up buying from somebody else, REI is a few social rungs up the ladder from me. Try Bass Pro Shops
instead. This is a link to their Big and Tall department. They have 500 items listed. Be careful they don't sell you an RV and bass boat along with the shirt.
Lightweight rain poncho makes good sense, too.