Hard drive size

I am running Windows 98 platform, not SE. I have two hard drives in my computer, the first one is an 8 GIG partitioned in the middle, the second one is a 20 GIG set up as an extended Dos partition, but it won’t let me use the whole 20 GIG hard driveas an extended Dos partition. It does let me use 19 GIG of the 20, in the extended Dos partition, why can’t I use the whole 20 GIG as an extended Dos partition?