My smart-ass answer is that if you don't know, you probably don't need or really want RAID0. That said:
For working with really big files sequentially (such as video editing, or streaming big files over a network), you generally want the largest stripe size.
If you are doing 'general-purpose' stuff with the drive (installing an OS, applications, and/or small media files on it), stick with whatever the default is (usually 16 or 32K). It is unlikely to make a huge difference anyway.
If you know for some reason your software accesses very small pieces of data (for instance, some database servers can be tuned to always try to read/write in aligned 4K or 8K pieces), you should match up the stripe size to what the software will use. But if you were setting up something like this you would know this already or should have hired someone who does.