Gearing and muscle development

Generally I think it makes sense to use a slightly lower gear freewheel than fixed.

As you have the option of stopping pedalling down steep hills.

I ran 52:19 (on 27" wheel) which is about the same as yours and I loved that ratio.

If you like 52:18 freewheel I would not go any lower than that for fixed. Would suggest you stuck to the same or slightly higher, but 52:17 is too much of a jump.

Well im actually running that ratio on 27's. But I will stick to what I like(for now anyway).
Any suggestions for a (cheapish) rear fixed hub to replace my current one. The make is Suzue, they are high flange (7cm across). I would give spoke length but I have no idea what it is(I didnt build the wheels) and have no idea how to work it out.
Cheers.