I myself always buy the biggest, most of the time I put the designs on tote bags, jackets or aprons. Most machine will let you cut the design down by 20%., or enlarge it 20% Also depends on the size of your hoops. I use my 8 x 11-1/2 99% of the time, but it depends on your hoops. I normally only use my 8 x 8 or 8 x 11-1/2.
You can PM with any question that I might be able to help you with.