I won't get into the different aspects of metal ferrules - that's a discussion for another time - but I will say that all metal ferrules are made slightly over sized by .001" or more. It is up to the rod maker when mounting the ferrules onto the rod to lap the male slides of the ferrules down to size. A very small amount of metal must be removed from the male ferrule in order for it to fit into the female. This is done so that the rod maker has control over the fit of the ferrules. A new pair of ferrules off the lathe looks like this:
To properly lap a male ferrule to size, the rod maker must spin the ferrule. The small amount of metal that must be removed needs to be taken off evenly on all sides of the round male slide. The metal itself can be removed with different grains of wet/dry sanding paper, special lapping files, of a combination of the two. In the photo below, I've temporarily mounted a male ferrule onto a drill bit to spin it at high speeds while I lap down the slide to the size I need.
Now, about ferrule fit:
Some folks like a very tight fitting ferrule. They want to have to pull a little harder to remove the rod sections & they want to hear a loud, distinct "pop" sound when the two rod sections come apart. That's absolutely fine, if that's what you like. For others, they don't really care so much - as long as the ferrules hold well & the rod doesn't come apart while fishing.
In my experience, when making rods for clients in general, it's always best to make the ferrules fit a very small bit looser. There are many reasons for this.
Mostly it's because as you use your fly rod, the ferrules will accumulate a small amount of dirt on the surface. This is environmental stuff, like grease from your hands, elements in the air & water, etc. This stuff over time builds a very small film on the surface of the metal & so the ferrules will occasionally need to be cleaned - wiped down with a soft cloth, wet with alcohol.
If the ferrules are fit a hair looser, it will give you more time between cleanings. On very tight fitting ferrules, you must always keep them clean until the ferrules have been used for quite a long time.
One more reason for a looser ferrule fit is the old habits of folks who have fished with nothing but synthetic rods. You should never twist metal ferrules - ever. It's a great way to tear the bamboo strips in your rod apart or to break the glue bond holding your ferrules onto your fly rod. You can get away with twisting the sections together if they don't line up correctly on modern synthetic rods, but not with the Nickle-Silver ferrules on bamboo fly rods. In my experience, I've seen too many angler twist their rod section to line them up when putting a rod together. If the ferrules are a wee bit looser on your bamboo rod, it might just save you a broken rod - if you catch yourself doing the twist in time.
You might have heard that when metal ferrules make a clicking sound, it means that they are loose & should be repaired. That is not true. It simply means that the male ferrule is moving slightly side to side inside the female while the rod is being wiggled. The only time you need to worry about loose-fitting ferrules is when they start to come apart during normal casting & fishing. Metal ferrules will eventually wear out from use, as it is a friction fit that keeps them together, but not for many years & quite possibly longer than you or I will be around. Save your money & don't have a set of ferrules replaced only because they make a clicking sound. Wait until there's actually a problem with the ferrule fit.