Gearing and muscle development

Any suggestions for me? I have massive legs anyway, I always have really not really for any particular reason just do.
I am currently running 52-18 ss but planning to change to fixed, I like the ratio but not sure what fixed would be like on the legs.

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.