How do I pick the right size when buying online if I can't try things on?

The key thing is don't look at some abstract size, but at that specific store or brand's size chart, since they all differ. Measure yourself with a tape measure (chest circumference, waist, sleeve length) and compare it to their measurements instead of relying on your usual M or L. If you're still unsure, go for the larger size - it's easier to deal with than paying extra to exchange something that's too small. And definitely check the return policy before you buy: a good store will let you return or exchange even worn items.