I don't know if your question was directed at me. But, I'll answer it
I don't think the actual population number is that significant. Anything from 1000 to 5000 would probably fir the bill.
Here are the factors I DO think are important. And again, this is only one man's opinion. Everyone will have pluses and minuses. And, we all need to choose what is right for us and our families lifestyles
1) It should be a farming town. Meaning, it is surrounded by farmland and the businesses in it tend to be supported by farmers and agriculture as an industry. This is because you want to be where there is an abundance of food and land to grow it, as opposed to an environment where food is scarce.
Remember, these farmers have been working this land for generations. In a SHTF scenario, they will have a surplus capacity, because food is not going to the cities. They are going to sell to those who live in their community FIRST.
It also helps that this type of community, by design, is going to have a surplus of storage, machinery, seeds, tools, etc. They are everyday items in an agricultural setting.
2) It should not be within a day or so's walk from a major interstate. When SHTF, city dwellers will jump in their cars and hit the road. After the gridlock ends and gas runs out, they are going to abandon their vehicles in search of food and shelter. The "offramp" towns will be the easiest pickings for these folks. Hopefully, by the time they get to your town, their number will be severely reduced by violence, starvation, exposure, etc...
3) It should be of sufficient size that it includes a number of non-agricultural businesses i.e.- auto repair, grocery, bank, hair salon, doctors office, butcher, as well as a small amount of manufacturing.
These businesses have most probably been supported almost entirely by the towns people and surrounding farmers. So, they are economically insulated.
Not only do they provide a diversified knowledge base. They are also excellent income opportunities. Unless you are a farmer, you will need to use what marketable skills you have to barter.
4) It should be in a region with plenty of rainfall and a nice long growing season. In a SHTF scenario, you will want to make things easier,not harder. Plenty of free water and 2 harvests on your garden will mean less hunting and/or working for a farmer or business to barter for food.
Those are my initial thoughts, anyway.