What size unit should I install in a 1900 square foot 1 story house? 4 ton??

There is no correct short answer to your question. You need to do, or have done a manual J heat gain calculation. There are numerous free or low cost programs available online. Here is one I've used & been happy with: http://hvaccomputer.com/

Florida? Alaska, Under trees? Good insulation? Low E windows? 1 story or 2? Basement? Need lots more info