shrinkage
 

i want to finish with a 6 1/2 x6 1/2 block i want to machine embroider on them how big a piece should i start with

Personally, I would start with 9" blocks. I would definately wash the fabric before cutting into blocks because of shinkage possibilities....Be sure you use plenty of stabilizer under your blocks to minimize any drawing up and "ruffling" when you do your embroidery work. When you have the blocks completed, then you can cut them down to the size you need for your quilt. Better safe than sorry! This may be an overkill with the size, but you never know!
Good Luck!

Large enough to fit securely in your sewing machine hoop, unless your machine has the baste feature. I would start out with a fairly large piece if you are doing many different blocks. Draw your 6.5 block on the fabric, then center each design as you come to it. Then cut them out to finish your quilting project.