What size tarps should I keep on the trailer?

If you buy 8ft, buy the 3 pc. ones so you can use center one for a steel tarp.

 

^ yep. If your just doing legal, sticks and bricks flatbed loads, get a 3 piece with the center tarp boxed out with D rings and a 16x24 with D rings on all 4 sides. Those 4 tarps will nearly cover everything with minimal extra tarp.

 

I will say 8' is a better bet since you can use it to cover 6' load also.

 

For more years than I care to say, I only used standard 8ft lumber tarps. Used them for just about anything you could think to tarp. Just make sure you get 4 rows of D rings or loops all the way around.