I'm sure you can order 4X10 sheets of Lauan plywood but it will be costly. Since your roof is going to have pieces butted together anyway, using 4X8 sheets with a 5" piece spliced in there will be fine and will be much cheaper. I would suggest using a narrow Crown Stapler to attach the Lauan to the rafters. If you don't have one, Harborfreight sells one for under $30
Crown Stapler I have a Porter Cable Crown Stapler that I always use when working with Lauan but for a one time project, the Harborfreight gun is good enough.
2020 Silverado 2500HD LT, CC, 4X4 6.6 Duramax
2021 Grand Design Reflection 311BHS
I asked him to do one thing and he didn't do any of them.