Bleach will indeed hasten the deterioration of the material. People hand-washing and rinsing the insides of a PUP cannot remove all of the bleach and therein lies the problem.
I have had good luck with Iosso Mold and Mildew Stain Remover. It is designed for woven materials (primarily sails for sailboats) and is non-toxic, doesn't smell (important when you are inside a PUP) and is biodegradable. I googled a supplier and have been quite pleased with it.
Pop-up sizes are based on "box "size" which means the length of the lower hard wall section of the PUP where it touches the frame underneath.
They come as small as 6' and range up to as much as 16'. A very common size is a 10' box, which is what I would suggest you look at.
You may have stated elsewhere, but how capable is your tow vehicle? PUPs can very quickly outweigh the tow capabilities of a number of passenger vehicles.
Knowing what you can safely tow is actually where you need to start FIRST, then see what other criteria, wants/needs and options fall within that constraint.
It can actually be quite fun to go shopping for a PUP, see lots of units and sit them, dream and so forth. But you need to do your homework first on what it takes to tow a PUP safely.
You will get into things such as what your state requires if the PUP is above a certain weight, such as its own brakes, etc.
Those won't necessarily steer you away from a particular purchase but you may need to buy a PUP realizing that you will need to add brakes to it to be compliant with the law, if it applies.
Anyway, good luck with the search. Short of renting a PUP (which people say to do all the time but the availability of PUPs for renting in my parts of the world is zero), you just have to sit in a few and imagine what it is like to live inside it for a weekend in the rain: how would you manage cooking (if you decide to cook inside - some chose not to), how do you shuffle the suitcases around, where and how to store food, etc, etc, etc.
If your tow vehicle can support heavier weights, then by all means look at 12' boxes as these generally afford more amenities or make the same ones a little more comfortable. My PUP is actually a highwall unit with a 14' box, which is a major upgrade from our first one that was a standard wall 10' unit. They feel like totally different worlds. The first 10' PUP felt cramped with just the 4 of us and no place to put anything. The 14' high wall is like a cavern with too much storage space - something you never hear a PUP owner say.