Gauss cannon base hit chance IS linear to size. Steve has said that -- for example -- although 1/6 size Gauss Cannons claim to have a 17% base hit chance in the description, it's actually 16.666...%

This means that absent any other modifiers, six size 1 Gauss Cannons will *average* the same number of hits as one size 6 Gauss Cannon. Since most bonuses and penalties are multiplicative, these averages will remain constant in those cases.

Crew Rating, on the other hand, is NOT. Highly skilled Fighter pilots/crew will ADD their flat bonus to hit chances, resulting in (for example) hit chance multipliers of 105% for one size 6 Gauss Cannon and 21.666...% * 6 for six size 1 Gauss Cannons (equivalent to one shot at 131%).

Therefore, with Conscript crews you want the largest size GC that you can fit, whereas with trained and/or experienced crews, the smallest. Multiple smaller GC also have advantages in the 'at least X hits' category, redundancy, and -as noted by Michael Sandy - reseaerch/Tech costs. They suffer in damage absorption (you have watch total HTK closely) and potential overkill (for example, when you are trying to cripple engines prior to boarding enemy vessels).