I don't think the size of the incubator is the factor that determines what thermostat you use. Wattage and amps of your heat source are gonna determine if your thermostat can handle the" juice" put through it. Wafer thermostats have done a superb job of controlling the temps in thousands incubators over the years. With good insulation you can "get away with" using a lower wattage element.
Below is some good advice from BYCers
rebelcowboysnb says "A wafer thermostat is rated for 22 amps... At 110 volts that's over 3000 watts so it would work. I have not seen any digital thermostats that handle over 500 watts."
You must also have good enough circulation to insure that you don't have cold or hot spots in your incubator. Your wafer thermostat will control temps based on what the temperature is at the location you have it.
An element that uses a lot of amps might require you to incorporate a relay to protect the thermostat. JimnTer below is answering a question concerning whether or not my wafer thermostat could handle the strain I was gonna be putting on it. The micro switch he's talking about is the little switch on the wafer assembly that actually shuts off and starts the heat source.
JimnTer helped me with this answer to a question I had; "How many amps is your microswitch rated for? It will say on the side of it. 2 X100 watt bulbs draws about 1.67 amps at 110 volts. If your switch is rated lower than 1.67 amps at 110 volts, you will need a relay, but I would say if its rated for less than 2 amps you should add the relay. If you are pushing the switch near its maximum capacity, it will shorten its life. If the switch is rated for more than 2 amps at 110v there is no advantage to adding a relay."