Short-stroking refers to using a subset of the data space on each drive, only using the outer sectors. The idea is to minimize head movement and improve seek times. There are two methods. One involves very low-level access to the drive, to tell it that it has less space, and I don't know how to do it. The second is done after you build the RAID volume; just allocate a small data partition at the beginning of the volume and use only that partition.
If by "install stuff on the other amount that my os isn't in" you mean the rest of the space that you didn't use for shortstroking, you can't. If you use it, the drives are no longer short-stroked. If you just mean the space in the above-mentioned partition that isn't your OS, just put files on the C: drive and that's that.
to "what amount of gb do I need to partition by," traditionally shortstroking runs about ten percent of the total disk size.
With no offense meant, your questions suggest that you have read some more advanced threads on setup but not basic information on allocating storage and installing an OS. If I am correct in this guess, then RAID 0 is probably not a good place to start.
The key questions are 1) Why do you want to shortstroke and 2) Are you aware of the serious risks to your data from RAID0 and 3) Are you aware that RAID0 does not improve, and may worsen, seek times, which has a bigger effect than the throughput rate? Plus the biggy, 4, "What are you trying to accomplish that couldn't be done with the simple use of a single drive?" With the answer to these, the more experienced may be able to point you in the correct direction.