The system should see it as a 20GB hard drive. Ultra10 supports both Solaris 8 and 2.6. Specs as follows:

Up to two 20-GB, 7200-rpm EIDE hard disks
Operating System Solaris 8
Solaris 7 Hardware: 3/99
Solaris 2.6 Hardware: 5/98
Solaris 2.5.1 Hardware: 11/97
UltraSPARC-IIi, 440 MHz
Cache 2 MB (external)
Memory 168-pin EDO JEDEC, ECC error correction with 4 slots, 128 MB up to 1 GB (minimum factory-configured system is 128 MB) with DRAM speed of 50 ns, and DIMM sizes of: 32, 64, 128, and 256 MB (installed in pairs)


Where are you seeing it as a 8GB drive? Did you double check that the drive you have installed IS a 20GB drive?

You should be able to boot into single user from cdrom and go into format to see the size of the disk drive.