When using raid0 on two HDDs of different sizes.
The raid0 size is limited to twice the size of the SMALLER HDD with the remaining space on the larger drive unusable.
You can not set a partition on the larger drive and use that partition as part of a raid0 setup.
This is incorrect. You can set partitions as part of array. But that is fully dependent on your raid controller and or your raid software.
So, if your raid management tool support it, you can create a 2x500GB partition (actually use about 501GB and 499GB since block alignment might give you less if you choose 500GB). Use the larger partition in your RAID array and the remain 499GB for whatever you choose. Keep in mind that your RAID performance will still be limited by your slowest disk and performance will be further impacted if you write to both partitions at the same time.