My advice is to get at least a 6-quart or larger cooker, even if you are cooking only for 2. Since you can only fill at cooker 2/3 full–you need to allow room for the steam pressure to gather), a 6-quart cooker is really equal to a 4-quart pot.
An 8-quart cooker allows you maximum flexibility in your want to do duplex and triplex cooking, such as layering potatoes on the bottom, a meatloaf on the potatoes, and a packet on foil-wrapped carrots on top.
For a second cooker, it’s nice to have a 2 1/2- or 4-quart size. A smaller cooker is great for making risotto and small batches of veggies.
Some manufacturers offer a large and small cooker with one lid. These are handy if you don’t plan to be making two pressure-cooked dishes at once.