Since itohy's patch is really nice, the followings are just
digression. :-)

> > > > I don't know about "most people", but I tend to use -s=<half-phys-mem>
> > > > for sizing an MFS /tmp, and make sure /var (which is always on a
> > > > separate partition) is large enough to hold two crash dumps (ie, 2 x
> > > > phys-mem).
>       (+2*size of kernel image)
> 
> I have a symlink from /var/crash to somewhere else; 260MBytes of disc space

NetBSD's default installation doesn't create the symlink. :-)

> > I think historically it is recommended to make size of /var/tmp larger
> > than size of /tmp. And I always do this setting.

> Out of curiosity: When & where was this recommended?

Early 1990's.
As Robert Elz said, both sort and updatedb used /usr/tmp for big
temporary files in those days.
Hmm, I might be already oldtimer...

I'm really surprised by reading your pointing out that sort uses /tmp
as temporary file in these days. :-)

> Most programs (tar, pax, compiler stuff, many, many packages - nearly
> everything except updateddb.csh?) use /tmp as default to store their
> temporary files---often larger than the average stuff updatedb.csh

Tar and compiler stuff always use /tmp. It wasn't larger than about
15MB (at least in those days).
--
soda