gypsymama has sizing for ordering which could also be applyed to making your own.
of course it really depends on what carries you like to do. I have a short wrap (3.88yds) and sevral long ones (5yds) I use them all depending on what carry I am going to be doing. fabric and length both matter.
my best suggestion is to to to walmart, look at the $1 table for cotten gauze, and get 5 or 6 yards. you can use it for some time (if you dont wash it) without hemming. so if its too long you can shorten it with relative ease. then if you ever want to buy a wrap just measure the one you made and youll know what size to order.
wraps are also very secure even without tying a whole square knot. some people will just tuck the ends in. I useually (with a wrap or mt) will just tie once. like the begining part of tying your shoes, before you start the bow.