I want to buy an outdoor bag when I travel. How big is it? Is 30L enough?

I don't think it's enough. Hiking bags are generally 45L for men and 35L for women, which are of regular size. It's best to solve everything with a big bag outdoors. Don't pack a big bag in your left hand, pack a small bag in your right hand. It's too troublesome and inefficient. Buy a package. If you are not satisfied, you can reserve space. It would be a tragedy to buy small ones.