Is a 60 amp breaker adequate for a 240v sub-panel to my garage?
You mention two different things:


A specific appliance - 10,000 W electric heater
A subpanel


Do you actually want/need a subpanel?

If you only need to connect the heater (i.e., you have plenty of power already available in the garage for lights, receptacles, etc.), just run a circuit (wire &amp; breaker in main panel sized appropriately) and you're done. For a 240V heater, that would likely be just two hots + ground.

If you actually want to connect a subpanel, then:


The wire &amp; breaker in the main panel need to match - breaker protects wire, but should be sized for a larger load so that you can put other stuff in the subpanel
The subpanel should (code experts can speak up as to whether this is "should" or "must" - I'm not sure) have a disconnect mechanism, which can be a main breaker ("main" of that panel, but in the subpanel, not the 150 main panel) any size equal to or larger than the breaker in the main panel that feeds the subpanel. So you might get a panel that includes a 125A main breaker and use that even though the feed might be 100A.
To connect a subpanel you should use hot/hot/neutral/ground so that you can have 120V loads as well as 240V loads in the subpanel.
The total of all the breakers in the subpanel can be more than the feed - e.g., you could have 60A 240V for the heater and 6 x 20A 120V for lights, receptacles, power tools, etc. for a total of 120A @ 240V but as long as you do not have everything running at full load all the time.