Well this becomes kind of a wierd situation.
You can cut the lengths to be any length you want, as long as its around a foot long. If you cut between the zig-zagged black stipes the chances are you will cut one of the black stripes and have problems, or the flex watt will not work and over heat the heating elements possibly further causing problems.
I would not try to cut anything like a 1/2' length. It just doesnt seem safe. If your trying to cut a 1' length down to like 11" then well you can cut it close to the end.
All in all, i wouldn't cut the black zig-zagged heating strips or in between them. Just folding the flex watt might be safer.
If you still want to try to cut the flex watt, just Email BigAppleHerp and ask them if you can do it. They have been very helpfull when i've dealt with them via email.
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Later,
WhisQuila