The largest IDE hard drive has been the Maxtor 536DX (4W100H6). Some weeks ago, Western Digital released their 100 GB drive as well - even at 7,200 rpm. With the per-platter capacity of 40 GB, it would be no problem to assemble drives with a capacity of 120 or even 160 GB. Maxtor and Western Digital are preparing to launch such drives soon.
There were several hurdles to overcome with the IDE interface: 504 MB, 2 GB, 8 GB and later 32 GB. The next limitation is caused by the LBA (logical block addressing) of the UltraATA standard which works with 28 Bits. At 228 Sectors with 512 Bytes each, the maximum disk capacity is 137.4 billion Bytes (or 128 GB). On September 3, Maxtor and some others introduced UltraATA/133 , which is not only meant to speed up IDE performance up to 133 MB/s, but also will remove the 128 GB limitation.
Although UltraATA/133 is widely supported and certainly will replace ATA/100 for the time before Serial ATA finally arrives, not the whole industry supports it. While VIA, SiS and ALi are going to support the new protocol in their next chipset generations, Intel as well as Seagate and IBM prefer to wait for Serial ATA. At least, ATA/133 is downwards compatible, which allows to run the new drives with an ATA/100 or ATA/66 interface - at reduced performance.