I can't recall now, so I'd have to measure when I'm there. Usually it's 4" apart but also depends on the length so it'll all look proportional.

Thanks for your quick response. I am currently having our stairs redone, and the proportion is exactly what I'm worried about. I chose a square top baluster, but the 1-3/4 inch base seems to be too big. I'm considering going back to the pin top with 1-1/4 inch base. I would greatly appreciate a second opinion! :)

I'd go with 1 3/4 top and base and skinnier middle of the baluster, so to visually create more space between them. Also, I'd try to push them apart if that works in the allowed space... Otherwise, it might get too busy.
Hope I helped. Thank you for your question.