The original 28-bit Logical Block Addressing scheme allowed for 268,435,456 sectors of 512 bytes each, for a total of 137,438,953,472 bytes.
The max size you want to know depends on whether you want to treat kilo, mega, giga as units of 1000 or as units of 1024.
137,438,953,472 bytes = 137,438,953 kilo-1000 bytes = 137,439 mega-1000 bytes = 137 giga-1000 bytes
137,438,953,472 bytes = 134,217,728 kilo-1024 bytes = 131,072 mega-1024 bytes = 128 giga-1024 bytes.
Additional note: Typically, GB stands for giga-1000 bytes, and GiB stands for giga-1024 bytes. But the last time I looked in Windows XP, they display hard drive sizes using the GB abbreviation, but their calculation uses giga-1024 bytes, which is confusing. I've seen a website or 2 mistakenly claim that 120,000,000,000 bytes "formats to" 111 GB, when in reality, 120,000,000,000 bytes iS EQUAL TO 111 GiB, all because Windows XP uses GB when they meant GiB.
So even though XP says your partition is 128 GB, you may really be at 128 GiB, right smack against the upper limit of 28-bit LBA.
Hope this helps.