What size suitcase should I buy for a 10 day trip?

1) northern or southern california?
2) time of year? (i.e. in winter you need a sweater or two)
3) do you plan to do laundry while there?

Should be fine if you do laundry.

If you can’t put in a carry-on bag then you don’t need it.

A standard carry on suitcase would be plenty, I think, but this is coming from a person who went to Greece for 4 weeks with only a carry on.
I’m all for doing laundry, packing things you can wear more than once and packing pieces that you can mix and match together. Since it’s a casual trip, it should be no problem.

Welcome to Fluther.

Different people do “casual” in different ways. What kinds of activities will you be doing, and how do you normally like to dress for those activities? It might also help to know whether you’re male or female.

You also need to take into account how much shopping you plan on doing. If you anticipate getting souvenirs or heading into shops, then you need to buy a suitcase that will not only give you wiggle room in packing what you need for the trip, but will also accommodate whatever you end up bringing back. Keep in mind, that you are allowed to bring a carry-on as well as a personal item (such as a backpack.)

I could easily do a 10-day trip with a carry-on bag and personal item, but I am an expert folder and relatively low-maintenance.

For my last trip i bought a backpack that opens all the way like a suitcase. It is so much more convenient to wear a backpack than roll luggage behind you.