50L is a pretty good target for a 2 to 4-day pack. Presuming you don't have a bulky bag, sleeping pad, shelter or a lot of clothes, it's enough volume. I like the ability to strap gear outside (e.g., shelter, pad, etc.) because the food, at least at the beginning, can take up a lot of space. A big, stretchy back pocket is a plus, too.

Because there are hundreds to choose from, narrowing it down is a chore. I'll only suggest never overlooking fit in search of a bargain.

Good luck!