Added... It's self evident that donors need to be larger than the frame being cut for. But how much larger? According to @Chris A Hardaway, "it depends on the way they are beveled. Normally I want 1 mm total width, and 1 mm height. (So 0.5mm extra per edge)... But if they have any odd step-bevel (like Oakley wires) then 4 mm total!"