Size is very personal. Some people can travel full time in a 16' travel trailer full time and be very happy. Others need a 40' 5th wheel and still feel cramped.
Personally, I would want to get at least up into the 27-30' range. Particularly on a rainy day with the 3 grand kids...but as long as it's a short time, you can make it work in a smaller unit.
Lots of options in that price range. I wouldn't consider a unit without slides and for the most part they are reliable.
If you are new, find a mobile RV tech and pay him a couple hundred to go over the unit once you think you've found a winner. The single biggest issue with all RV's is water leaks. A good tech will be able to find evidence of prior leaks.