You got a bargain.
Swapping old tanks is a good idea, though some dealers don't like it, best to do it at big stores, they don't look, just swap.
I would avoid anything smaller than 5 gallon (20 LB) size. Most refillers charge a premium to refill small tanks, so the bigger the tank the better the deal on propane.
You can buy many different sizes, if you pay to get a whole pallet of tanks delivered. The most common is the 5 gallon (20 LB), next the 7 gallon (30 LB) size, then it gets fuzzy, tall tanks, short fat tanks etc.
Propane tanks got a lot more expense this year, adding 40-50 percent to the price, the increased steel cost they say.
About the largest portable tank for me is a 15 gallon (50 LB), it weighs just over 100 pounds when full. If you have a lift gate truck, or other means then maybe you can handle larger portable tanks. I am not aware of any propane businesses that will come and refill a small tank, think you will have to go to them. l
Last,... tanks are only filled to 80 percent these days to allow for expansion, so your 5 gallon tank only holds 4 gallons.