Backpack Size

I’m planning to backpack Southeast Asia for 3-4 months. I was wondering what size backpack everyone recommends? I’m thinking of bringing 2 because I plan on doing the Ha Giang Loop and and overnight excursion in Laos and I heard people bring smaller bags for that?

For a smaller bag I have a personal item sized backpack from Amazon I used for study abroad travel in europe, but it’s not waterproof so would you recommend getting something else?

For reference I don’t want to overpack a bunch, but I also don’t want to be totally minimalist. Any advice is appreciated as I’m a terrible packer :)