|"I can't edit the drive size in the BIOS as it's ghosted. "|
You can't edit the drive size in the bios yourself in any case. The bios detects the total size of the drive whether it has data on it or not, if the drive detection settings are correct.
If it's the Sony Pcv-rz32g you installed the drive on, it would not have a drive size limitation bug in the bios version that would prevent the 40gb drive from being detected as it's full size - it came with an 80gb drive.
Go into the bios and make sure the connection the drive is connected to is set to Auto detect by the method Auto or LBA.
Make sure you have the jumper set on the drive correctly:
DiamondMax D540X Ultra ATA 100/133 - Jumper, CHS, and Install Guide