Disclaimer- This is only a very basic guide, please consult a licensed electrician for specific applications. Tool damage from undersized or poor quality power devices may void warranty.
The size (wattage) needed depends on the tool and inverter, generator or transformer.
First to clarify terminology- an inverter converts 12 (or 24) V DC, usually from a vehicle into 120 Hz AC (household current). A generator converts a fossil fuel (typically gasoline, diesel or natural gas) into household current, and a step down transformer converts one voltage into another (typically, 240V to 120V- to use a North American tool in Europe etc.).
Generally anything with an electric motor (drills, saws, etc.) is going to require roughly 2-3 times the tool's "amp" rating for initial start up. Some tools that can soft start (VSR drills) require less initial power than an air compressor, which are notoriously difficult to get going.
To figure out wattage- multiply the amp rating by 120 (in North America).
For example, a table or miter saw rated at 15 amps will require around 1800 watts continuous, thus a 2500 watt generator, inverter or transformer would be the absolute minimum, with a larger unit being ideal. Also keep in mind- inverters, generators and transformers have widely differing designs and power quality.