Need I remind you that 6-bits bytes are artefacts of the past ?
Use only natural powers of two :
1, 2, 4, 8, 16, 32, 64, 128...
There are countless great reasons to stick to that. Mainly because if you don't, you end up with all kinds of weird stuff.
It's OK if you reserve a 2^n space and you don't use all the bits, though.