Todd,
Derrell Poole's plans show the following side board dimensions:
1897 3-board car, 2x10 upper boards, 2x12 bottom board
1898 4-board car, 2 1/4x10 boards
1902 4-board car, 2x10 boards
Not sure where he got the dimensions for the first two classes, the 1902 car was likely measured at Central City.
I chose to use 2x10 boards on my models abuilding.
Jim
Jim Courtney
Poulsbo, WA