A tape measure is needed to measure fibers during set up and for measuring the braid during braiding. For this reason I recommend that you have a tape measure that can measure up to 3 ft. (1 m).
This is a tool that most people will have in one form or another and they all have their advantages and drawbacks. Some of the ones available at hardware stores may be too heavy to include in your traveling toolkit. The type that is commonly used for sewing, although easy to use in the studio, could be less useful in the classroom. I find that a retractable craft tape measure is the most versatile and a good addition for a traveling toolkit.