Believe it's 2" by 4" (or roughly 50mm x 100mm - can't remember exact metric equivalents). This is what I have from reading all the various sources about building your own cellar board, but I ain't built one so don't go on what I say.
I do plan to build one though, probably about the same size as you have, maybe a little longer though. Ain't got a clue about supports sorry, I don't have that luxury since I'm living in rented accomm and plan to make a free standing one...
Best of luck with it though.
EDIT: I think you will need vertical supports not diagonal! i.e. create a box with 2" by 4", then vertical stiffeners spaced some x inches apart and screw your plywood on this. And then mount this at an angle. Believe 40 degrees is a good angle, and trigonometry is used to dictate length of board according to your constraints on where you can fix board etc.