— Burt Kaliski Jr. (@modulomathy) September 8, 2013
The largest primes of length 2 to 7 bits are 3, 7, 13, 31, 61, and 127.
As it turns out, the largest primes of length 2, 3, 5, and 7 bits are also the largest numbers of those particular lengths. This is because the primes are all Mersenne primes: primes of the form 2p-1 where p is also prime.
Interestingly, when n is composite, 2n-1 is also composite. In particular, if p is a divisor of n, then 2p-1 is a divisor of 2n-1. This is why the largest primes of length 4 and 6 are not also the largest numbers of those particular lengths.
It is not necessarily the case that when p is prime, 2p-1 is also prime, though it works for the four examples here.
The reason that there are no one-bit primes is that the only 1-bit numbers are 0 (which could technically also be considered a 0-bit number) and 1. Neither is prime. At one time, mathematicians considered 1 to be a prime, because, like other prime numbers, it is divisible only by 1 and itself. However, 1 is even more special: It is the multiplicative identity. For technical reasons, many properties involving prime numbers work out better if 1 keeps its special status.