I figured out a way around this with my Gigabyte EP45-UD3P system board. It was only letting me build 898GB partitions with my five 2TB drives in the BIOS, but I was able to build a 7.5TB RAID 5 partition using the Intel Matrix Storage Console. Follow these steps:
1. Turn on RAID in the BIOS. DO NOT build the RAID sets using the BIOS tool.
2. Load the latest version of Intel Matrix storage manager (64 bit - assuming you're working with a 64-bit OS given the partition size here)
3. Build your RAID 5 stripe using the tool.