I have a 30 and a 24 and it works well. I'm considering getting another 30 to replace the 24, but the downside is it would take up a lot of room on the desk and possibly even be harder to use because you'd have to turn your head more every time you want to look at the second monitor.
If you're currently using a 24 and just debating about getting another 24, then I'd say go for it. There's no reason to get anything smaller than a 24 especially when they can be had for so cheap anyway.