In short... No, they don't have to be the same size.
You would be limited to double the size of the smaller drive though for raid0
So, if you added a 100GB drive to the already existing 200GB you would have 200GB worth of total space rather than 300GB.
In my opinion, I dont know that it would be worth the risk.. you essentially are going to be running 2 drives and doubling the changes of a minor/major hardware failure. This may or may not end up in data loss... but it would most certainly take time to recover data. And you would be gaining speed during prolonged reads and writes... not necessarily initial file accessing times. That will, in most cases, take longer for 2 drives than just the single 200GB. But for reads and writes that take a long time... it would be faster, sure.
Increased reads over long time transfers
Increased writes over long time transfers
2x the hardware ware/tear risk
Slower access times than that of one disk... (this isn't read time... just the time it takes to FIND things on the array)
Limited to 2x the space of the lower capacity drive
On that laptop, I may try it for a while but would most likely go back to a single drive or mirror.
EDIT: @ Tenenbaum: no, you're absolutely right...in your scenario SubGum would have 400GB total
I'm slower at typing than you though, clearly...
2nd edit: SubGum, just checked your drive... it's a 7200 (in case you didn't know) I also found something about upgrading your BIOS on that model... but, you may want to look into it, I didn't because it's long and I'm sleepy.
Here's the link though The Official P-7811FX BIOS Version 9C.08.00 Download Is Here - Now with RAID! - Notebook Forums and Laptop Discussion