Yes, generators are rated by the watts they can handle.
So a 5000 model generator would be 5000 watts capacity.
A microwave oven would be about 1000 watts for example. Or a 100 watt light bulb 100 watts.
But most important is wheels! Especially if you need to cart it around. Some of those construction rental generators can be quite heavy and don't have wheels.
Then if you are using something higher wattage like 1000 watts and using a long extension cord, be sure to get a good "construction type" 12 gauge wire extension cord. I imagine they would rent these.
If you have a high wattage thing like an air compressor on a long extension cord, and the extension cord has a small gauge wire, then the air compressor might just "hum" and not even start-up!