Sub-panels are pretty inexpensive to start with, so go bigger than you think you'll ever need. I'd say at least a 6 space panel, 8 would be better. If, for whatever reason you needed to upgrade (maybe an EV charger?) later, a few dollars extra spent now for a decent sized sub-panel is way cheaper than upgrading a too small one later.
Again: rules for sub-panels:
- because it's a detached structure, it needs it's own ground rods.
- because it's a sub-panel, the neutral and ground must be isolated (not bonded, ie: not connected) The ground wire from the house (or wherever the main panel is located) will be connected to the ground bus bar in the sub panel, along with the locally installed ground rods.
- It's OK to have a sub-panel with a capacity greater than the feed to it.
There is a ton of good information regarding sub-panels here on SE. Do a few searches and you'll learn everything you need to know.