I have a 98 volvo s70 and i dont know the engine size how do i figure out the difference between 2.3 or 2.4

All S70's have a 2.4 liter. There should be a numbers like 2.4 or 2400cc or something like that. The 7th and 8th vin digit is the engine. 55 is a B 2454 S engine. If unsure call the dealer parts dept and they can tell you, based on the VIN number.

2.3 liter for a T5 version Turbo highly tuned for performance.
2.4 liter non turbo standard engine.
Look it up in Wikipedia. Then have the explanation and information.