I’m rebuilding a BMX as a bit of a project. It’s a “standard” size freehub on the back. What is the smallest gear I can put on there? What is “microdrive”? What is the best ratio for general faffing about?

what will u be riding? the factory basic is ither 44-16 or 25-9(cassette not freewheel) for the fasion victems in the middle u can get about a 36-13ish
on a standerd rear u can go down to a 15t ish freewheel then it starts going into smaller thread
id stick a 44-16 on and have a play with ratios fron there i. run 43-16 quicker acc but not less top end but im a spinner