Once the array has been created which in your case will be seen as one big 1.5TB drive, you can then partition it per normal.
More importantly, why are you putting 10k/15k drives (assuming that's what those 300GB are) into RAID5?
Those higher RPM drive are normally utilised for high IOps usage involving small writes to the disk, RAID5 or 6 is a complete performance disaster in under such usage not to mention long recovery time during rebuilds, RAID10 is what normally used with those disks for redundancy. If you just wanted higher sustained speed ( a video production server perhaps) then striping a bunch of high density 7200rpm would've made more sense with much lower $/GB.
In these days with cheap $/GB 7200rpm disks, RAID with parity is mostly associated with nearline data archiving instead as there's many performance & integrity downfalls to using them w/o proper implantation.