Is a 2.5 ton central AC system too small for a 1700 sq foot house ?

2.5 ton is probably too small. We can't really answer without knowing where the house is and how it was contructed.

Yes, you do need a larger unit. Contact a certified AC contractor to have your home sized for the correct type unit. I would recommend at least a 3 ton 16 seer variable speed unit. My choice of brands are American Standard or Trane.

The size of AC unit depends of isolation on: roof, walls, windows, doors...

So if your entire house is well insulated you will need at least 3.5 tons but a tecnician must measure your house, check the isolation rate of all elements and count all the heat sources for give you a size

A 3 ton unit would be better unless you live in a really hot and humid climate. Then I would look into a 3.5 ton unit. 4 ton is to big.

I must accept as true with geeka...All of the manufacturers are usually the identical, most have 10 yr components warranties. Even Trane has thier problems with leaking evaporator coils. Any mechanical object is field to failure, but most a/c issues are the effect of a poor install.

Heck yeah that tonnage is way undersized.