I haven't used the installer for a while but I'm pretty sure it will offer to make use of the largest free space on the disk. Let the installer format the partition, always accept the defaults unless you know better.
I always create a single OpenBSD partition on the disk before running the installer by using
GPT fdisk, which is available for Windows or Linux. A single partition of type 'a600' is all that is needed.
OpenBSD will create it's own swap, no need to make one for it.
Also relevant:
https://www.openbsd.org/faq/faq4.html#Partitioning