Since Raid 0 uses a striped array both of the drives used have to be of the same size.
It will combine the overall size of the drives and merge them into, or present it as one drive in windows.
Example: Two 500Gb drives in a raid 0 array close to 1Tb of space after a format and install of windows os.
You will get a slight increase in read and write performance by setting up a raid array with two drives.
But they will never match a single SSD drive in through put on reading or writing.