It would be helpful if you provided more information such as what type of circuit the rheostat is being used in and what the rheostat is controlling. I have to assume the rating is 20 watts due to the power being dissipated. You can't simply replace it with a unit having a lower rating unless you can show that the 20 watts i s overkill. However, assuming a 20 watts is the required rating, you may be able to reduce its size by replacing it with a fixed resistor and a lower ohm rheostat. This is possible if the full range of the rheostat is not being used. For example, if you replace it with a 15 ohm fixed resistor and a 15 ohm rheostat, then the maximum power in the rheostat is now only 10 watts. Thus you can use a smaller unit. However, the downside is that the variation in resistance would now only be from 15 to 30 ohms instead of 0 to 30 ohms. This may be acceptable depending on the actual circuit.