Partitions are not assigned any particular letters -- with the exception of a (root), b (swap) and c (entire disk), the rest of the partitions (through letter p) are available for use as you desire.

On the root disk, the two partitions 'a' and 'b' must be created. The installation process will not proceed until these two partitions are available. 'a' will be used for the root filesystem (/) and 'b' will be used as swap space.

After a little thought, we decide to create just enough partitions to allow the creation of the recommended separate filesystems (/, /tmp, /var, /usr, /home) along with a swap partition: 
	wd0a: / (root) - 150M. Should be more than enough. 
	wd0b: (swap)   - 300M. 
	wd0d: /tmp     - 120M. /tmp is used for building some software,
                               120M will probably be enough for most things. 
	wd0e: /var     -  80M. If this were to be a web or mail server,
                               we'd have made this partition much larger,
                               but, that's not what we are doing. 
	wd0g: /usr     -   3G. We want this partition to be large enough to
                               load a few user applications, plus be able to
                               update and rebuild the system by source if
                               desired or needed. The Ports tree will be here
                               as well, which will take almost 140M of this
                               space before ports are built. If one was
                               planning on building many applications from
                               source using ports rather than pre-built
                               packages, you might want a lot more space here. 
	wd0h: /home    -   4G. This will allow plenty of user file space.
How much disk space is required for OpenBSD? Obviously, the answer to this question varies tremendously based on your use of the system. However, these numbers can be used as a starting point: 
	(root)             100MB
	/usr               250MB (no X) or 400MB (with X)
	/var               25MB
	/tmp               50MB
	swap               32MB
Those are minimum suggested filesystem sizes for a full system install. The numbers include enough extra space to permit you to run a typical home system that is connected to the Internet, but not much else.

For your first attempt at an experimentation system, one big / partition and swap may be easiest until you know how much space you need. By doing this you will be sacrificing some of the default security features of OpenBSD that require separate filesystems for /, /tmp, /var, /usr and /home. However, you probably should not be going into production with your first OpenBSD install.

A system exposed to the Internet or other hostile forces should have a separate /var (and maybe even a separate /var/log) for logging.

If you outgrow a partition, you can allocate a new partition from your unused space, duplicate your existing partition to the new partition, change /etc/fstab to point to the new partition, remount, you now have more space. http://www.openbsd.org/faq/faq10.html#DupFS

How do I duplicate a filesystem?
To duplicate your filesystem use dump(8) and restore(8). For example, to
duplicate everything under directory SRC to directory DST, do a:
	cd /SRC; dump 0f - . | (cd /DST; restore -rf - )
dump is designed to give you plenty of backup capabilities, and it may be
an overkill if you just want to duplicate a part of a (or an entire)
filesystem. The command tar(1) may be faster for this operation. The format
looks very similar: 
	cd /SRC; tar cf -  . | (cd /DST; tar xpf - )
There are several reasons for using separate filesystems, instead of shoving everything into one or two filesystems: Security: You can mark some filesystems as 'nosuid', 'nodev', 'noexec', 'readonly', etc. This is done by the install process, if you use the above described partitions. Stability: A user, or a misbehaved program, can fill a filesystem with garbage if they have write permissions for it. Your critical programs, which of course run on a different filesystem, do not get interrupted. Speed: A filesystem which gets written to frequently may get somewhat fragmented. (Luckily, the ffs filesystem that OpenBSD uses is not prone to heavy fragmentation.) Integrity: If one filesystem is corrupted for some reason then your other filesystems are still OK. Size: Many machines have limits on the area of a disk where the boot ROM can load the kernel from. In some cases, this limit may be very small (504M for an older 486), in other cases, a much larger limit (for example, 2G, 8G, or 128G on i386 systems). As the kernel can end up anywhere within the root partition, the entire root partition should be within this area. For more details, see this section. A good guideline might be to keep your / partition completely below 2G, unless you know your platform (and particular machine) can handle more (or less) than that. 
spc@ruby[~] $ df
Filesystem  512-blocks     Used    Avail Capacity  Mounted on
/dev/wd0a       158574    47858   102788    32%    /
/dev/wd0d       158638        2   150706     0%    /tmp
/dev/wd0e       158638    68608    82100    46%    /var
/dev/wd0g       693614   308070   350864    47%    /usr
/dev/wd0h      1305660      114  1240264     0%    /home

spc@ruby[~] $ cat /etc/fstab
/dev/wd0a /     ffs rw 1 1
/dev/wd0d /tmp  ffs rw 1 2
/dev/wd0e /var  ffs rw 1 2
/dev/wd0g /usr  ffs rw 1 2
/dev/wd0h /home ffs rw 1 2
/dev/cd0c /cdrom cd9660 ro,noauto 0 0 

OpenBSD partitioning convention is that 'a' is always '/', 'b' is swap
and 'c' is the whole disk.

/dev/wd0a      100M	/
/dev/wd0d      400M	/tmp
/dev/wd0e      4GB	/var
/dev/wd0g      2GB	/usr
/dev/wd0h      5GB	/home

root@diamond[~] # df -h
Filesystem     Size    Used   Avail Capacity  Mounted on
/dev/wd0a      502M   36.8M    440M     8%    /
/dev/wd0h      164M   18.0K    156M     0%    /home
/dev/wd0d      251M   13.7M    225M     6%    /tmp
/dev/wd0g      1.2G    422M    748M    36%    /usr