I used all 2x6 and it got me exactly within width of framework with about 1/8 to 3/16 between each board.... Trailer had some really bad quality cedar (I think) at first and I replaces with quality Douglas Fir... Went to traditional lumber yard and they let me sort through stack to find pieces I wanted... I use a "deck" sealer on them to protect from weather and trailer is out in yard 365....
My deck is 82-1/2 wide and it took 15 2x6's and no trimming... I uses same width/pattern that I was replacing...
Used all new trailer deck screws to secure new boards....
Ironically I found a pack of deck screws at industrial hardware that has exactly 15 screws, just what I needed, amazing...
Hint: When you have all the boards off frame, mark on top of outer frame rail where cross members are under deck so when putting screws back in, crossmember is easy to locate...
I used 2x6. I stacked and clamped them in the barn for several months to dry before installing. I then installed them as tight as possible. I now have small gaps, just what I was hoping for.
Those screws are called trailer floor screws i believe. Try to find them at least zink coated. The black ones wont last long in PT wood. Also you dont need PT. I have a center deck on my open car trailer thats normal 2x6 thats 8 years old. I still park my mini excavator on it.
I haven't had to replace my deck yet as I treated it with a 50/50 mix of linseed oil and ATF fluid. Got excellent penetration and it is not sticky or slick. Current deck is doug fir of various width. Treat it every few years using a pump sprayer.
For lighter-duty trailers, 5/4" x 6" PT deck boards work well. My old "woods trailer" used to have 3/4" PT plywood. When the plywood died, I replaced it with the deck boards and like them much better. They seem to be stronger, and the gaps let water drain out, so the boards seem to be lasting longer. I spaced mine with a 16D nail to create gaps, which widened a bit at the boards dried out. It's been working out well.