I used 2x2's for my subfloor base. Gave me less height than what 2x4 would have, and still suited my needs. After building and being (ALMOST!) done with my TD, I would have used 2x3's on the long frame side laid down on the 3" side of the board. The 2x2 did not leave me a lot of play to drill the holes for the mounting bolts to the trailer frame, and an extra 1" would have made my life easier.
If your trailer is factory built, taking measurements will be fine. If it comes in a box you have to put together, wait before building further and get your trailer put together. I have found box kits are all a bit different when assembled.
When it comes to tools, I used a square, a 5" trim saw, an impact driver, and a drill driver. Of course tape measures and various strait edges. Those were the tools I had, so that is what I worked with. Job could have been done much easier with a table saw and a shop saw, but I don't have either in my garage!

I used 1/2" plywood all the way around my build, floor and all, but I covered my interior walls with acoustic carpet, not luan or similar so I could afford the weight.
Hope my input is helpful!
Neil