What size unit do I need for a 2500 sq/ft house

What type of central cooling unit is the most economical

Not an AC man, until one shows up, would say between 3.5 and 4 tons. Wait for at least a second opinion.
Also check this while waiting:
Efficient Home Cooling (http://www.aceee.org/consumerguide/cooling.htm#reduce)

Well first of all, is all 2500 sf above ground? And is it all living space or some storage? Assuming it is all above ground you will need roughly 112,000 btus of heat, and I got 3.33 tons so I would go with a 3 1/2 ton unit. Also if this sf is all above ground and if you have a two story house I would recommend going with two, 2 ton units one on each floor.

Actually to be any way accurate you should do or have done a manual J to take the homes exact construction into account since any other method is nothing but guess work. Wall thickness and insulation,window size and type,number of doors,attic insulation and many other items need to be accurately calculated to get the proper size equipment. Failure to do this can cause many problems especially on the A/C sizing and the moisture removal during A/C operation.