I am looking at a trailer that doesn't have a sticker on it with the GVWR.
Is there any other markings or way that I can tell what the GVWR is on it?
It's a single axle.
Does tire size affect GVWR?

Thanks.

If the tires are original, you can get an approximate value. The tires(trailer tires) have a weight rating on them. Normally, the rating of the two tires added together will be a little higher than the gross weight capacity which includes weight of the trailer itself and loaded boat. If you know the manufacturer name, they will be able to give you a definitive answer. If they are not the original tires, I would not trust any number you derive from them. Good Luck and remember to open up the bearings to assure they are ok if it's a used trailer.

I would not trust the tires one bit. Check out the weight capacity of the axles.