You're unlikely to be able to find a new pata/laptop IDE drive so I wouldn't be suprised if your limitation is the size of the drive you can get rather than anything else. They simply won't make em any more.
Considering the limits for older systems were in terms of hundreds of gigabytes (128 GiB initially, then 128 PiB for ATA 6), that wouldn't really be an issue.
I know the transition between PATA and SATA happened around the 30gb size mark. So whoever upgraded it probably just picked a similar size drive to be sure.
That said, your SSD's probably likely to be a better idea as far as performance goes, and you'd be getting a new/old stock drive anyway. I'd consider increasing storage other ways. An external HDD might be a simpler option, even if you're stuck with USB 1.1 interfaces.