Ruth, Standard tires on both the Scamp 13 and 16 are 13 inch tires. Many users have upgarded to 14 inch tires for increased load capacity.
Changing from 13 inch tires to 14 inch tires requiring changing rim sizes.
Most trailer tires have the designation ST. ST stands for Special Trailer and are designed to be used only on trailers. This rating limits trailer speeds to 65 miles per hour.
Usually 13 inch ST tires come in two sizes ST175/80R13 and ST185/80R13. The ST175is capable of carrying a load of 1360 (2 tires 2720 lbs) and the ST185 is capable of carrying a load of 1480 lbs (2 tires 2960 lbs). These load capacities are defined at a tire air pressure of 50 psi.
Like 13 inch tires, 14 inch tires ST tires come in different sizes and correspondingly higher load capabilities, upto 1870 lbs (2 tires 3740 lbs).
Remember larger tires require larger rims and may require minor fiberglass modification.
As Floyd said the size Scamp is important to know because it helps us define the weight
of your trailer. If you have a Scamp 13 any of these tires should be adequate.
I have a Scamp 16 that weighs 2400 lbs and I use Goodyear Marathons, size ST185/80R13. I have been using these tires for 5 years without any issues.
Hope this helps and please let us know the length of your trailer. I bought my tires at Discount Tires. 5f you buy your tires locally you can also have them grease your wheel bearings while there.