RAID 0 setup does not contain full disk space from slices
You can't fix this. The size of a RAID is always a multiple of the smallest volume used in the setup.

The chunk sizes (stripe sizes, interlace sizes) as well as the number of chunks have to be equal on all used volumes. Usually the chunk size is 64 kb. These requirements are determined by the size of the smallest disk and limit the size of the resulting RAID volume.