As can be observed from the various sample graphs in pskmod documentation, the generated symbols have a magnitude of 1. So unless you are using a different basis function than the typical sine/cosine, it should already be providing you with a signal with an average transmit power of 1.
To get a signal of length 256, you can code it like this:
Mod = 4;
N = 256;
x_n = randi([0 Mod-1],N,1);
s_n = pskmod(x_n,Mod,pi/Mod);
