I am making a obw for a gift and I want to set them in squares with sashing....I know this is a dumb question but how do I figure out what size to cut the corner triangles to make the hexs square ?

This is tricky to work out as they are not HST's. If you made your OBW's as per the book, I would perhaps try this on one of them and see how you go. 3 1/2"squares cut diagonally to make 2 triangles, sew these on and then you will need to square them up.
Maybe someone will come up wih a better solution.

Ty woody....yes I went by the book.....although I am cutting my triangles bigger then I have before....I just dont want a lot of waste

If you just add corners, a OBW will not be a square. The corners can be made from rectangles that are approximately a 2-1 rational.

If you want a square, I suggest that you draw a full size pattern with the shapes you want, trace your extra pieces, add the seam allowance and use that as your template.

The first book has instructions for hexagons and octagons (triangles are different proportions, and the number of layers changes). You can add triangles to the octagons to make a square, but as Glassquilt pointed out, adding triangles to the hexagon version will give you a rectangle, not a square.

I figured it out thanks ladies

Just a suggestion, why don't you do the 4 patch posie instead? You get the kaliedoscope effect without having to mess with trying to figure out how to make a hexagon a square. You can sash the posie squares and they look really nice.

Lol yeah I should of done that but I already did it lol,

Ahh, one of those hindsight things.