3 x 3 x 1/4 on mine. It works well
3x3x1/4 is what I was gonna use but then decided against it altogether.
4X4 1/4 and I have one on the back as well as one down each side.. It looks a bit funny but I set them at perfect 90 degrees to each other.. I can just drop in two joints of pipe clamp them down and have a prefect 90 degree joint..
It has worked out well for me..!!
RC
I have used a 3 x 3 x 1/4 for years, works great.