Forged pistons depending on the silica content will not expand as much and will require tighter clearences. Although as already said these should come with the pistons.
You'll notice that some engines with forged pistons clatter like diesels from cold, this can be because they are low silica content pistons and take ages to expand, but normally it's down to poor machine work.
Be careful if using coated pistons. When I was building my MR2 I had mixed opinions about ceramic coating, then read a lot of horror stories (coating flaking and causing hot spots). The SR20 DET as with the Toyota 3S-GTE is fitted with oil squirters in the bores. These are designed to spray cooled oil at the underside of the piston crown to cool it.
The ceramic coating creates a thermal barrier and stops heat from being absorbed in to the piston. This heat has to go somewhere so the combustion chamber and valves take it, the squirters are not affective any more.
With higher combustion chamber temps the risk of detonation becomes a big threat. Be careful.
