Philip,

Having looked at the tartan you designed in the other thread (nice tartan, BTW) I have to say it looks like a symmetrical tartan to me. Maybe you can post a thread count for us?

But yes, whether a tartan is symmetrical will affect the sett size. For example, let's say your tartan pattern is represented as ABCDE. Well, in a normal symmetrical tartan, the pattern would go:
ABCDEDCBABCDEDCBA

So, to get from "A" to the next "A" would require a jump of 8 letters.

But now let's say you had that pattern woven as an asymmetrical tartan. It would run like this:
ABCDEABCDEABCDE

Now, to get from "A" to the next "A" in the pattern only requires a jump of 5 letters. So the sett repeat is now smaller, because you have not reversed the pattern.