I have a 1.5L Hawkins, and it's too small. I can barely get a baking potato in it, then get the lid on. The lid is made to go inside the pan so you get a safe seal. With a potato in the pan, getting the lid in is difficult. Generally, the 1.5L size is too small for most recipes for two persons without regard to having to get the lid in to close it. My cooker says it should not be filled more than 2/3 its maximum capacity, so that leaves me with 1L of food and water in the unit.
My next pressure cooker will be three liters.
Your mileage will vary.
Here's an REI pressure cooker:
http://www.rei.com/product/750389[NOTE extraneous to the original post: Pressure cookers are not marvelously fast. On mine, you put the stuff in you're going to cook, add water, then put the lid on without the vent weight. Heat on high till steam escapes freely from the vent. Put the vent weight on the vent. Continue heating on high heat till steam escapes from the vent weight. _Then_ the timing starts on the cooking, and you can lower the heat. With all that, it does cook faster than non-pressure cooking, but it's not the 4 or 5 minutes they say in the ads. It's only the 4 or 5 minutes after the water's been boiling for long enough to release steam. Then, surprise, you have to let the cooker cool down enough for the seal to break.]