When OpenBSD installs it is writing a list of partitions. These partitions have a letter as a name. (about from a to p).
The size of these partitions is all right for most purposes, but not always.
Changing the partitions size is difficult because it conflicts with working parts of the operating System.
The best way to avoid problems is to edit the partition table when installing OpenBSD.
One of the questions during install is about the partitioning.
OpenBSD writes a table and asks what to do.
You could accept, edit or write a new one.
I did edit the existing one. The editor is very helpful.
As partition a and b and c are more or less default I started to delete all partitions from partition d upward. The command in the editor to delete a partition is d.
After that add partitions (command is a) one by one.
The editor does ask you to give the right size and to write the mounted part.
Before the start I did write a note with the layout for the partitions. And I did write down the mounted parts like /usr, /home, /var, and like from the first table.
The command to safe and close the editor is q. If the editor does notice mistakes it will -mostly- not safe the changes.
Do not do this after reboot, because that will destroy your install.