I am 5'8" and have ranged from a VERY skinny 135lbs in college to 184 and muscular. What works for me has been:
IWB mostly. 4:00 to 5:00 always, and with a single mag pouch. I can get away with a Large size tshirt (since they are built large around the waist area already for bigger waisted guys). For OWB you will need to take more effort to dress around the gun.
Gun: Glock 26 for carry may be ideal for you, glock 19 for classes. Buy all holsters for Glock 19 and use them for both.
Kramer IWB #3 with their horsehide belt or a Wilderness nylon belt (nylon is not as stiff but the infinite adjustability is key when you have less squishy bits to be compacted. Especially when you are out eating or sitting for a while and then moving). Kramer also makes wicked good OWB holsters but they are a little more high visibility than say the Raven for this job.
Mitch Rosen has design principles DOWN. Not as gee wiz fast for classes, long wait, but outstanding and the cant is always set perfectly for how the holster is to be worn. Little tricks like having belt loops offset from the holster are oustanding for those who are thinner.
CCC makes a great IWB Kydex and can fit to your specs. Great prices too. Highly recommended
If you have to do OWB and kydex, go with Raven Concealment Phantom. They are bent just enough to fit around your waist properly.