Is there any type of giant clam that will fit in my 20 gallon tank?

If you can provide enough light and good water quality for it then yes you can keep a giant clam (Tridacna maxima) in a 20g tank for a while. However, after 10 years or so it might need to be put in a bigger tank. They are slow growers but can get big if you keep them alive.

Squamosa,derasea,croecea, of course the maxima too. They are long lasters but don't skimp on the lighting! 150 watt metal halide at least.

Agreed, Tridacnid clams need LIGHT! They're shallow water species adapted to very high light levels. In the wild many even live in tidal pools and are out of the water and rained on by freshwater... But if you don't give them light, they'll do very poorly. A Derasa or Crocea would be sustainable long-term in a tank that size as they rarely get much over 6" in shell size. Squamosa, Gigas, and Maxxima get HUGE! Clear over a foot in shell size in their old age

As well as dosing of having high calcium as there shell requires this to grow.

Indeed, they do guzzle calcium and carbonate

Any idea how long they take to get that big ? I'd like a big clam in my tank.....

when I get it set up

not meaning to hijack

I need to read a bit more before I figure out for sure what kind, though. I don't want a fast grower. Something that takes years and years would be nice

Think a decade before they get that big. Initial growth is fast, but tails off as the clam ages.

Thanks