This looks like it will be a fun quilt to put together. I seen no need for sashing since the white will make all your blocks stand out alone and just pop! lay your quilt out on the floor or on a design wall to get a better idea of the size. Since it is not sewn together up will need to subtract 1" for every 2 blocks laid up next to one another to get an idea (not accurate) of the size of the quilt. Add or subtract blocks to get what you want.