4"'dual is the the best one I think simply because it's a dual.

I'm using a Fletcher too, space is a problem for this DD. I have already bought a new DD,
waiting for my crew to be all level 20 before moving to the new ship.

Not quite.

You can mount whatever gun your gunner has the level to operate. If you HAD a
level 24 gunner on board, you could, for example, use a 1x5"/38 Type A.

Lvl 21 is the highest gun lvl on a fletcher.

personally i prefer the dual 4"s on the fletch. i like the fast reload. i'm not too
worried about accuracy so i pump as many shells into a target as possible.

man my b i was thinking ddx for a while. 6" singles is the best for fletch

You mean 6"/47D. guns are the highest level guns you can mount on the fletcher.
However, I would recommend the /L variant which has slightly more range.

With the L variaint you will be able to carry 3 binds of 6 inch shells per turret. That
means 150 rounds per gun. (its more than nuff )

6" duels are the biggest and best on a fletch. butthose dont come until lvl 22+

Because guns take up space, if you jump into a sommers or anything bigger, cl>ca>bb> you
get more gunspace for ammo and different types of guns (Move up from 4" -12" - 16")
Thoughout your whole lvl's 1-120.

And to answer your questions, yes, 4" duels should be the biggest guns, unless u put
singles on it, could be 5" or 6" singles.