An SX 25, which would likely have a BIOS from 1991 / 1992 would not be able to handle any drive over 504MB without either a drive overlay or an LBA extension ROM (typically mounted on a network card)
If you get a network card and install a suitable size ROM on there with this ROM installed: http://mame.spludlow.co.uk/Machine.aspx?name=lba_enhancer
Then you can have drives over 504MB, probably even 2GB. You just need to set the BIOS settings to CHS 8:8:8:
Set hard Disk Type: "47=USER TYPE"
I've tested it and it works quite well. Though it's possible it causes a problem with detecting ATAPI (CDrom) drives.
Alternatively, to use drives bigger than 504MB, you can use Ontrack drive overlay, but that stops the disk being compatible with regular partitioning so you can't just take it out and load files on via a PC.
Personally, I would just go with 256MB CF cards on the older machines (pre 1994) that don't support LBA, but that did get easier for me since I got a crazy deal on 40x 256MB cards recently. Up until then, I was collecting up ISA network cards to use for the LBA enhancer ROM method.