Subpanel breaker size and wire size
Currentwise, I think a 60A supply breaker will suffice.  You use #6 copper or #4 aluminum wire for this. There's a case to be made for either one. Copper can be spliced in smaller boxes, but it's stupid expensive and it invites a dissimilar metal issue on the panel lugs (which are typically zinc plated aluminum).   If you run this cable in conduit, and you use THHN/THWN-2 wire, then you might as well use a 70A breaker since the #6Cu/#4Al 60A wire is good for 70A when it's in THHN. 

Panel-wise, we need to have a conversation.  The most difficult problems we see here (the ratio between simplicity of project vs cost to do it) is "Panel Full" situations.  "My panel is full, how do I add a circuit?" often means a $1000+ panel replacement for a simple circuit add.  In practicality, that means the on-demand heater, table saw, or EV charger simply does not happen. 

So, you have to size up your own house. If you've got a 40 space panel that's half empty, this may not be much help.  But not knowing all those details, our default reco would be for something on the order of an 18-space panel - that would give you a lot of expansion room for future circuits.  For instance if you needed room in the main panel, nearby circuits could be rerouted here.  The cost differential is not that large, and the protection it gives you is considerable. 

The subpanel does not need a main breaker, and the bus rating is just a rating, like the 130 mph rating on your car's tires. So there's nothing wrong with feeding a 125A or 200A panel from a 60A breaker for instance.