Length If you are making a braid for a specific project, always start with 3 times your finished length. Anything over can be used for a key ring or scissors tag etc.
Adding beads The beading thread should be twice the length of the other threads.
Thickness To estimate how thick your braid will be, take the threads you are going to use (same number) and twist together. This will give you the approximate thickness of the finished braid.