So, it sound like you are attempting to make a quilt without any pattern. It is hard to determine fabric needs without having the finished size of the quilt. If your "unfinished" blocks are 6 1/2" than you finished blocks will be 6".
You can take pieces of 6" square paper & lay them out to get the dimensions of the quilt you would like to end up with. Once you determine the size of the quilt top, you can then calculate how much you need for backing.
Do you want to have an outer border? If no outer border, just add 1/2" to width & add 1/2" to length. Also, I cut my backing pretty close, but if you are a new quilter, you might want to add an additional couple inches to the backing piece.