What size BB tool to tighten this Square taper BB?

I don’t know the frame brand. It’s a repainted aluminium track frame with a carbon fork. Looks quite good actually!

Anyway, I got the bike home and realithat not only was the wrong size seatpost installed (30.8mm post in a 31.6mm hole), but also the BB had a little bit of play. I can spin the BB with my fingers to tighten them. I figure I’ll pull the BB out to put grease on the threads at the same time.

I took off the cranks and am now wondering which size BB tool I need. I don’t have any calipers and holding a ruler up to it is tricky!

Any help will be greatly appreciated!