You chose the size kit you want.  Measure the size stretching board that you need, this is slightly larger than the image you will see in the frame aperture.  Add the 4 sides together i.e. 4+4+4+4=16 and this is the kit size you need.
Please note our smallest kit size is 4x4. 
REMEMBER to add the size of the actual frame you would like me to build for you in the Notes section on the order form.