A lot of that depends on your regs and your tow vehicle. Here in CT, I refused to take on anything over 10k because it requires either a DOT number or special waiver from DOT that you have to apply for and show non-commercial use, etc. And, while you certainly have to take into account what you're currently towing with, give some thought to what might happen in the future as well. Don't oversize yourself such that you have to have a heavier duty vehicle to tow with in the future.
As far as length, you'll have to sort of "figure that out" by seeing how it fits on your 16' trailer and kind of going from there.
The good news is that everyone selling a used equipment trailer believes they are made out of gold and price them accordingly... You should be able to recoup MOST of what you spent on it if you have to sell it outright. After you do, look around at the dealers in your general area and push them against one another to make them compete. I was going to drive to PA to buy a Big Tex 10ET-18 until a dealer in NH (MUCH closer) agreed to match the price. Be sure to ask what the MSRP is of a trailer and then figure out from there what you should spend.
A $5000 trailer (MSRP) should have a base cost to the dealer of roughly $2500-$2800 and a freight charge of around $200-$400 depending on where it's coming from. If you knew what I paid for my trailer, you'd be absolutely disgusted with what dealers regularly charge people to buy the same trailer.