What you want in the subpanel is enough spaces.
Get a subpanel that is large enough (in amps) to give you enough spaces to future-proof your house. Keeping in mind a few things:
- the way you are using double-stuff breakers right now, is not a viable long-term strategy. New work now requires AFCI or GFCI breakers, and those are not available in double-stuff, each circuit will need a full space.
- having spaces lets you do stuff you wouldn't otherwise do.
- Plug-in electrics (including engine-too types) are exploding in the auto industry (just go to The North American Auto Show or view their app) and we foresee that mixing with the smart grid too (your car backfeeds to help the grid stay up, and you get paid for that) - that could well be coming to your garage. So at least 2 more breaker spaces.
- On-demand hot water is making more and more sense, 2-8 spaces depending on setup.
- it may be a good long-term strategy to eventually make this panel the main panel.
- Some people want generators, off-grid switchable solar, or grid-tied solar. More spaces.
So it isn't wrong to buy a 42-space 225A subpanel. You can feed that from any breaker <=225A, including 150A, 100A, 60A, 30A and even 15A. The key is to think ahead to give yourself freedom to maneuver, rather than painting yourself into a corner.