Question about recommended Raid 0 stripe size windows 7

It probably does not matter.
A stripe is how much data will be written to one drive before an equal sized stripe is written to the other.
The value of raid-0 comes when you have an application that reads lots of data sequentially. Then, a read can start on one drive, and some concurrency can be had when reading to the second drive. Or writing...
In the case of video conversion, you will be reading an input, applying some processing, and writing the output. It makes more sense to me to have the input on one drive, and the output on the other. That way, there will be little access arm movement for either file.

I suggest you try it both ways, and see which works out better.