I'm not sure whether this is helpful, but on speaking to Bartolini: I had 6-string bass with 18mm spacing, so a total of 90mm at the bridge. The pickup inside was denoted as having 84mm 'sensing width'. Out of curiosity, I emailed Bartolini whether this was 'enough', and they said that it should be fine, and pick up all of strings correctly.
From observation, I can't hear any 'weakness' on the outside strings (low-b or hi-c), so that story seems to check out. Fine and balanced, despite the 'sensing width' seems to be narrower than bridge spacing.