Adding a 100 amp main panel
Some initial notes
You can run any amperage you please to the garage.
That thing you're running to the garage is on your side of the meter, so it's feeder not a service (that's why 310.15(B)(7) says &quot;service and feeder&quot;).
If the garage is a separate building (i.e. if the meter is NOT mounted on the garage building), then you need a local disconnect at the garage.  It does not need to be outdoors.  The cheapest way to get a local disconnect is to select a breaker panel that has a main breaker, the size of the main breaker does not matter.  Literally anything will do, so if you like a panel with a lot of spaces and those only come in 200A, that's fine.
The wire has to be actually rated for the specific known load that you are driving, or the load calculation you have done.  But if you don't have a specific load in mind or just want to exceed those minimums, you can run any size you want.
You must protect the wire with a circuit breaker, but you get to &quot;round up&quot; in breaker sizes.  If the wire is rated for 115A, you get to use a 125A breaker.  There's one exception: your feeder never has to be bigger than your service wire has to be (as per 310.15(B)(7)).
Feeder in these ~100A sizes should be aluminum - it's what any electrician would use because it's 1/3 the price of copper.  The panel lugs are aluminum anyway.  (aluminum lugs are the &quot;universal donor&quot;, they play nice with both copper and aluminum wire). So I'll be talking aluminum sizes.
Ready?
You don't need a breaker in the main panel if you simply bring over the full 200A to the garage.  Why would you do that? Well, what else would you do with it?  You don't have anything to power at the main panel if the only things on it are cameras and lights.
However the trick would be attaching 200A wires on the load side of the main breaker.  For this you'd need a main panel with &quot;through-lugs&quot;, or a &quot;Subfeed lug&quot; that clips in like a regular breaker, or an actual regular breaker that is 200A. They must be made for that panel; you can't use off-brand devices (except for things UL-Classified for your panel, but nobody makes what you'd need).
But if you can do that, then you can carry 200A from the main panel to the sub on 4/0 aluminum wires.  This is allowed because 310.15(B)(7) allows 4/0 for the entire service, and you don't need any bigger than that.
There's no need to enlarge the wire for voltage-drop reasons; it isn't far enough to matter.
If you want to carry less than 200A, e.g. to save money on wire or because there's no way to get lugs of 200A breaker... then simply use a breaker appropriate for the wire size (rounding up to the next available breaker):  #2 aluminum = 90A, #1 aluminum = 100A, 1/0 aluminum = 125A.