This question does not make any sense. In every version of UNIX that I know of, PID's are in the range of 0 through 32000 and thus are restained to 5 digits. And introducing a further restraint to, say, a max of 9999 would require access to source code. It is certainly not tunable.

What version of UNIX are you using? What makes you think that you have 7 digit PID's?

I think that it's very unlikely indeed that you really have 7 digit PID's. But if you do, remember that no one is forcing you to display them in base 10. The largest 7 digit base 10 number is 9999999. In hexadecimal that's 98967F.