4x8 or 5x8 trailer. plus what about fiberglass

This is my first post so bare with me guys. I want to build a simple yes thats right a simple or perhaps im just simple. I plain to build this thing in the spring I feel with cold weather already here and the garage full at the moment I should show patience and over come the urge to jump right in and make sure I know what im exactly in for anyway I know if I go with a 4x8 trailer I wont have to worry about seams in the plywood. So what happens if I use a 5x8 trailer I asume I will have seems in the wood. And I hear you use fiberglass for both trailer sizes to cover the plywood. I know I can build the thing Ive been around wood working for a long time as my father use to make custom furniture and Ive inherrited all his tools. So LOL
hopefully I have not confused you guys to much and you can give some advice. Thanks Aaron
