The most basic information you need is what is the towing capacity of your vehicle, and its max cargo load and max supportable tongue weight. You then need to find a trailer whose gross vehicle weight is below the max tow rating of the truck, and whose tongue weight (plus any cargo you plan to put in the truck bed), is lower than the max tongue weight capacity and cargo capacity of your truck.
You can find some of the specs on old trailers at the following:
The general rule of thumb is that the older the trailer is, the larger trailer you can get with relatively low weight, as they used to place a higher priority on weight back in the day (ie., in the 50's and 60's). By the time you get into the 90's, the larger trailers get pretty heavy.