My house is about 2,000 Sq. Ft, in New Jersey. We have Vinyl Siding.
The contractor suggested a 2.5 ton unit (30,000 BTU) based on teh
Windows, size, etc.

I wonder if thats large enough?

We currently have a number of individual units, and their total BTU is
around 45,000.
Even using 20* 2,000 SQ/Ft = 40,000 BTU

The contractor is reputable, and seems knowledgeable. I don't think a
larger unit costs much more and generally contractors will be happy to
sell a more expensive unit, as they make a bit more.

I'm just concerned that 30,000 is not enough.