I would like to make some 6 1/2" pinwheel blocks. I want to use the 2 square method using 1/4" seam around the 4 sides then cut diagonally to make 4 half square triangles. My question is how big do I have to cut the 2 squares? Hope I have made this clear but am sorry if I have not. This is the first time posting and I have no doubt there is someone out there who can help me. Thank you in advance.

If my math is right it would be 2and 7/8"

I can't give you the answer on the size, but wanted to ask, are you sure you want you outside edges on the bias? I think you will get that if you use this method of construction.

I Agree. If you proceed with this method, which does go pretty quickly, make sure you starch the heck out of your fabric.
Do you want 6 1/2 inch FINISHED blocks, or 'raw' that would finish to 6" ?

When we were doing the pinwheel project a member, Ceil, came up with this formula that works!
"Whatever your block side is subtract .5 then multiply by 1.4 then subtract .5. "

So you would have:

Start with a 5.5 inch square
5.5 minus .5 = 5
5 x 1.4 = 7
7 Minus .5 = 6.5

Try it on some scrap fabric.
If your seams are a true 1/4" it should work.
If it is a little small try starting with 5 3/4" and trim finished block.
If it's a little big trim back.
HTH

Edit to add: Yes, as the others have said starch or sizing is a must before you cut out the squares.

:-D

Thank you all so much for your help. I will make sure I starch the fabric well.