Buying a Jon Boat
The first parameter to think of is what you are going to do with the boat. If the main reason to get it is to have little fishing boat that will be on a tranquil lake then there really isn't any reason to get a huge boat. If, on the other hand, you are going to have more than one passenger then it will be mandatory that you have a boat large enough to handle the extra people. It is dangerous and possibly illegal to force a boats capacity. If the boat is going to be a family boat or sort of party boat then you will want to get something a little more substantial. All manufactured boats will have a tag stating how many people the boat can safely handle and how much weight it can support.
If you are going to be transporting it each time you use it then you need to figure out how you are going to do that before you buy the boat. If you really don't want to buy a trailer or don't have a place for it then get a smaller boat. If you are going to need a bigger boat no matter what then you will need to start looking at trailers. This will also entail making sure that you have a vehicle that can tow the trailer and boat and having a hitch and the proper ball. Since jon boats are relatively light compared to their size your biggest challenge will be getting a trailer that can support the dimensions rather than the weight. You will need to know the weight of the boat you are interested in and the weight of the trailer to make sure that your vehicle can handle it.