I assume you are using the Intel RAID capability of your board, with the Intel RAID IRST driver?
This is Intel's description of RAID 10 from the help documentation available from the Windows IRST GUI:
It sounds like you wanted a RAID 1 + 0 as described here:
RAID 0+1: striped sets in a mirrored set (minimum four drives; even number of drives) provides fault tolerance and improved performance but increases complexity.
The key difference from RAID 1+0 is that RAID 0+1 creates a second striped set to mirror a primary striped set. The array continues to operate with one or more drives failed in the same mirror set, but if drives fail on both sides of the mirror the data on the RAID system is lost.
RAID 1+0: (a.k.a. RAID 10) mirrored sets in a striped set (minimum four drives; even number of drives) provides fault tolerance and improved performance but increases complexity.
The key difference from RAID 0+1 is that RAID 1+0 creates a striped set from a series of mirrored drives. The array can sustain multiple drive losses so long as no mirror loses all its drives.
Intel's implementation of RAID 10 is RAID 0 + 1, given this information:
Intel® Rapid Storage Technology (Intel® RST) — RAID 0, 1, 5, 10, Matrix RAID, RAID-Ready
The Intel RAID 10 volume size is 2/n, where n is the number of equal capacity drives used. So the volume size you get is normal for that RAID 10.
Intel's RAID is supposed to follow the SNIA standard for RAID. I don't see any one step method of creating the RAID 10 (1 + 0) you want. You could try starting over and create two RAID 1 volumes of two drives each, and then try to create a RAID 0 volume of those volumes. I don't know if that will work.