I've never wanted for a smaller trailer... That said, your axles limit what you can and can't do with a utility trailer. I have a 12 footer and it's plenty big enough. I can haul a couple yards of bark, a cord of wood, etc.
It has a 5600 pound axle under it, so getting two yards of rock is maxing it out.
That said, I have a 8' x 14' deck over trailer for my 18' cat. I've never wanted for less or more. It always gets hauled with one of my trucks, so space for extra stuff is never lacking.