Does anyone know what these values should be?
IDE devices (which are ATA5 or lower) have a command set with an 8-bit "sector count" field.
SCSI devices (which are SCSI10) have a command set with a 16-bit "sector count" field.
A sector has been defined (until recently) as 512 bytes.
By setting the MAX TRANSFER to a value of "0x1FE00" (255 * 512) prevents the scsi.device for IDE interfaces from creating malformed IDE commands.
While IDE commands support 8-bit sector counts, not all devices like them. Windows (and other operating systems) tend to transfer data using 128 sector commands. By setting your max transfer to "0xFE00" (127 * 512) you pretty much guarantee "sector count" compatibility with any IDE device.