I am planning on installing a Garage ceiling heater, which has a fan, and 240 volt coil electric heater. This unit is 26 amps, and 6000 watts, and will be installed in my garage, 60 feet from my main panel. Can you tell me what size wire, and breaker I need to feed this heater?
For a 26A nameplate load, Code will require #8 wire and a 40A circuit breaker. There really isn't any legal way around it.
Not what you asked, but by code you also must have 8 ft. from the bottom of the heater to the garage floor, because you have to avoid igniting any potential gas leaks from your car (remember, technically a "garage" is for a car, although few of use use them for that). If your garage has an 8 ft. ceiling, that isn't going to fit.
Google finds plenty of links. For example: https://www.thespruce.com/matching-wire-size-to-circuit-amperage-1152865
AWG 10 would seem to do. Once you pick the gauge, you must use the appropriate breaker for the wire. There might be, in your code some extra you must factor in. @harper points out, this is NEC 424.3b and adds 25%. This increases it to 8 AWG.
You can calculate the voltage drop at sites like http://www.calculator.net/voltage-drop-calculator.html
If I got your situation correct, that would only be a 3-volt drop. Looks acceptable.