While early editions of Windows 95 only handled 2GB partitions, most later versions handle large hard drives just fine and automatically take over all the disk space they can get. Most versions do not ask you how much they should get, as it’s obvious that anyone who is running Windows wants to dedicate their whole machine to it, right? You must use a tool such as fips.exe to resize your hard drive.
OpenBSD includes fips.exe in the “tools” directory under the release directory. The documentation included with fips.exe is fairly good, and Windows 9x is becoming increasingly rare among the people likely to be installing dual-boot systems, so we aren’t going to go into any detail on how to make it work. Just read the instructions and follow them precicely.
Remember, make your Windows 9x partition no larger than 7.5GB; you want to have enough room to get an OpenBSD root partition on your system!