My advice on large partitions holds to a point, but there are practical limits. The root partition must still be bootable; it must be FFS and not FFS2. Even though this is an expedient, and should be considered transitory, it is possible to run this way long term. There are security and operational implications to doing so. FAQ 14.8 discusses this in some detail.
I mention this because there was a thread on misc@ about an unbootable installation:
http://comments.gmane.org/gmane.os.openbsd.misc/189615