I bought an Odlo synthetic medium baselayer recently and sent it back because it was too tight. I then ordered a large, it came this morning, fits like a glove but not a tight glove, I'm wearing it now, so soft and cosy. I'll test it properly tomorrow.
I find that sizing is the biggest problem when buying clothes online, 'medium' can sometimes mean all sorts of different sizes.
The two garments I remember most were my Paramo Alta 2, I bought a medium but it was too big so I exchanged it for a small, and it was still too big, but not as big as the medium so kept it. I then bought a Marmot Precip, same problem, the small was big but I kept it.
I've bought boots online and to date, I've not had a sizing problem.
