I'd like some general advice here from anyone who has been in a similar situation, and has better ref-fu than I do.
I am running Skull and Shackles; currently the party are at level 11 and one of them has taken leadership as feat. I am not totally against the concept of leadership per se, and in the context of the campaign him having his own group of loyal pirates fits just fine.
However, in one recent encounter he picked up a large magic weapon from a cyclops and wants to hire a large cohort to use it.
I am not sure if a large creature as an ally would be overpowered. The large template adds +1 to the CR of a monster, but my gut feeling is that if they are on the same side as the PCs then reach etc. may be worth more than 1 level. There aren't any Large PC races (NB: we don't own or use 3rd party material) that I am aware of so I don't have anything to look at as an example of character level adjustment for size.
Does anyone have any experience of large PCs or cohorts and how they affect things?
I don't think they would be overpowered, at least if you keep in mind the HD/CR of the creature he wants as a cohort.
The only real advantage that the large creature will have compared to a medium creature is reach, and I thing there are much more powerful things that he could get with Leadership than a creature with reach.