On this page you will find some no-blathering honest information regarding sizes and deliveries in text, images, and also a video.
To the Video:
I am 5'9" (175cm) slim to average build, about a 40" (100cm) chest and I'm carrying a few extra kilos at the waste but not much...
Tees - Medium
It's a good loose fit across the chest, the bottom hem falls a couple of inches (50cm) below my belt and the neck isn't too tight. The sleeves fall just below the bottom of my biceps. If you want to show muscle you can roll them up.
So, the medium tee for my size will be good for most. My advice is if you are buying for a taller guy, or one with a thicker neck, or carrying more weight, I would go up a size. For the ladies, a medium would look good on anyone a from 5'3" to my height (5'9"). Go smaller or larger to suit.
The below image is a medium and not sized up
A medium with sleeves rolled up for an athletic look...
Below, lady wears a small, guy a medium
Tees - Large
This feels a little too large for me. It's a little looser around the chest and I expect would be good for guys of up to 44-inch (112cm) chest (and larger if you like to show some definition). The bottom hem falls to the base of my butt so it could look better on taller guys. But if you're a dude who likes the groovy baggie look, this will suffice. Sleeves fall to the inside elbow, and neck is nice and loose.
Hoodies - Large
I only tried the large as this is perfect for me. I think a medium would be a little small unless you have a very athletic figure. The hoodie comes down to just above my eyebrows, so that's plenty of room. The arms are a good length. The sleeves come down to an inch (25mm) or so above my knuckles. There's a good length of sleeve band to turn up if needed but I keep it as it is. The bottom hem is about the same as the large tee described above. If you are usually a medium and carry a little extra weight around the belly, this size won't hug around your midsection as it's very flattering.
For the ladies, unless you like things really baggie (first image), I expect that your normal size will be the right one to go for (second image). So the take home here? If in doubt, size up.
Image below - Baggie Look (go for at least an XL)
The below image is regular size i.e., not sized up.
Both the tees and the hoodies are warm, soft, and feel good quality. I have no complaints. In fact, I'm pretty stoked.