I just bought a RoadPro 2500 and i want to buy like a dorm fridge how many watts or volts size would work??

The correct size of inverter is an easy calculation and important to ensure long life from the inverter and the life of the appliances getting there power from the inverter. To calculate you take the volts and multiply by the amps which equals the wattage. Every appliance has a tag which must state the volts and amps. Locate the tag to find the information. Here is an example.

Under Counter Refrigerator 1.3 amps (120 volts x 1.3 amps = 156 watts)
Microwave (120 volts x 10 amps = 1200 watts)
Alarm Clock Radio (120 volts x .03 amps = 3.6watts)
TV (120 volts x .6 amps = 72 watts)

You now add all the watts together, since the above list could likely be running at the same time. There is a combine wattage of 1431.6 watts. In this example you would need at least a 1500 watt inverter. However given some unknown variables personally I would get a 2500 watt inverter. Less stress on the inverter, will lengthen the life of the inverter. Had the total been 1,000 watts a 1,500 watt inverter would suffice.

You must also remember that your alternator on your vehicle must be able to keep up with the power demand of not only your vehicle needs but your inverter needs as well.