As others have said, Harbor Freight, Northern Tool, and Tractor Supply along with about a bazillion other independents sell new small trailers. The Rebel will fit on a 4X8 or maybe even less, but that size is not good for much else then. 5X10, 6X10, and 6X12 are more popular because you can do more with them, but each one gets heavier to pull.
I prefer the 6X12 because my 4 wheeler or golf cart will fit on it with no problems, and I can carry 2 bikes. It is a little more stressful to pull since it is wider than the truck, and requires more attention and effort to stay in your lane. I had a 5X10 which was the exact width of the truck and a dream to pull, but it was too short for the golf cart and really tight for 2 bikes so I traded up.
My recommendation is to check Craigslist or other local sales avenues for a used trailer if you want to save some money. Auctions are a really good spot to pick one up cheap.
1999 Honda Rebel CMX250C
2003 Harley Sportster 883 Hugger
1998 Harley FXDWG Dyna Wide Glide
ALL IN! GO TIGERS!