You will see the screen below after running the OpenBSD installer and it will ask you to choose your disk.
You could have added other partitions by setting the root partition to 128M, and possibly added a /usr at partition 'f' using the remainder of the disk. You can choose your layout to be more advanced, but I meet many people who just want one big partition using the whole disk. You may now select your install packages, or sets as named in OpenBSD, and finish your install.