Nobody can tell you what size is right for you, nor what level of amenities is suitable. It all depends on your expectations and standards. That is especialy true for vacation use, i.e. a week at a time. You can cope with a lot of minor inconveniences for a week but the same things could drive you batty if fulltiming. Given that you are coming from tents and pop-ups, probably any trailer will look spacious and luxurious. I would suggest looking at something around 24-28 feet and see if you think it will be comfortable for you. An RV show is a good place to get a feel for what is available in various size ranges.
You are wise to buy used, especially since you aren't really sure what you want. There are a lot of fine, used trailers on the market, so take your time and shop around. Look on the internet as well as in local RV stores, even if you don't want to buy away from home. I's a god way to see what makes, models and types are available and what prices are being asked. You can also check used retail prices at www.nadaguides.com.
Your budget is also a key factor - I'm guessing you don't want to invest a fortune in a trailer that you expect to use only occasionaly. I generally recommend that people buy as much quality in an RV as they can afford, but "afford" is usually relative to the expecetd benefit from the purchase (unless you have money to burn

).
Do you already have a truck you expect to use to tow with? If so, let's start with the truck make/model/size/engine/etc. so we can help you determine how much trailer you can tow with it. That
may will put a finite bound on what sizes you can consider.