Very important. It needs to be 1-3/8. Some will say smaller or larger depending on whether the bees are making brood or honey comb. I made all mine the same. You may get a little bending or wave on the edges, but if you keep on top of it, it is correctable.
I should add for anyone building a TBH, that it may be best to keep the bar length for your TBH the same as a langsthroth bar. It will allow you you transfer the comb into standard equipment, or even sell to others (If they do the same). When they are all different "custom" build TBH's with varying bar lengths, you lose many possibilities and flexibility.