젖은 옷을 마른 가방에 넣어도 되나요?

Dry bags are designed to do exactly what the name suggests- keep the contents dry. But there is another meaning-isolate the wet or damp items from everything else. So you can of course put the wet clothes in a dry bag. This is a temporary way of store the wet clothes. Don't forget the wet clothes and dry them as soon as possible ;-)

드라이 백으로 옷을 빨 수 있습니까?

Now that the dry bag can store the wet clothes, it is also be used to wash them. Dry bag is designed to stop water from getting in that means they can also hold water. So you can also use it to wash clothes. But it is highly recommended to use neutral soap. Please don't forget to clean your dry bag with water after washing clothes.

옷을 보관하고 세탁할 때 어떤 크기의 드라이 백이 가장 좋습니까?

I use a 20L dry sack for washing clothes. This has no issues at all and it stores clothes well if it's dried completely even in wet rainy hiking and camping situations. I put a tiny rubber spiky ball which is a toy for small dogs into this 20 L dry sack and then put dirty clothes in and add enough water to move them around with a very small amount of bio degradable detergent such as Dr bronners soap. Shake it and knead it lightly with the dogs toy ball inside then throw away the dirty water and clean water to rinse. Repeat shaking and kneading it lightly, then throw the water away again. Wring the clothes out, then hang the wet clothes and the dry sack flipped inside out on a cloth line. When it's dried, fold the clothes, roll them up and place them neatly inside the same clean dried 20L dry sack. Now, I'm ready to put it into my backpack even when hiking in rain. My clothes stay dry and clean. I've learned this trick from one long haired Japanese hinter / camper who moves like a ninja on trails.