You’re probably looking at a 10-12’. Anything shorter and you’ll be exhausted and miserable getting anywhere. Longer and you wont be able to maneuver and transporting it is a PITA. You can still take a 12’ on rivers and in some moderate fast moving water, but no actual rapids. If you can find a place that rents them, try a few out at different lengths. Proper paddling form also keeps you from getting tired by only using your arms. You may know all that already, if so nm.
I’d also recommend one with a water sealed “trunk” area or at least get a waterproof bag (for sale everywhere kayaks are sold) for your stuff. I spent 7-8 hrs in a Pungo 120 last weekend that would be perfect for your use but the Wilderness stuff is expensive. This is a great thing to find used on Craigslist btw.