What determines speaker distance?

All formulas aside, where the center image starts to lose focus is the maximum spacing distance for YOUR listening space and position therein.

To add to Gs5556 comment - the further apart your speakers, in general, the larger the soundstage. Try STARTING at 7 feet and opening this distance up to at least 9 feet, until the center image loses focus.

I agree with both the previous posts and would add that you should also consider experimenting with different degrees of toe-in.

I own two pairs of ProAc monitors and their manual recommends toeing them in so they point directly at the listener so neither side of the cabinet is visible when looking at them. However, just as having the speakers closer together will help center focus so too will toe-in, but may also have the effect of narrowing the soundstage.

The set up I'm using is with the speakers six feet apart and my listening position varying between 6-8 feet from the front planes of the monitors. I have them toed in about half way between straight forward and pointing at me. Even in my small room this placement gives me the balance of focus and soundstage width I enjoy.

It all depends on your room and your speakers, but all of the above is good advice. Try checking out one of the set-up formulas (like the one on the Cardas website). They take your room dimensions into account. The speaker to speaker distance, speaker to side wall distance, speaker to rear wall distance, and speaker to listener distance are all part of a formula based on your room dimensions. Try using the formula as a starting point and then make adjustments from there. My Merlins also recommend a slight toe-in, but that would depend on your speaker's design.