Another consideration would be look at your timeout value for SMTP and work
backwards from there.  For example, if that's 72 hours and you want to be
able to withstand a worst case delivery outage (this totally depends on your
architecture) + growth, you'd want to have enough space to hold 72 hours
worth of SMTP messages + growth + system overhead + buffer space (in case
you're wrong about how much space you need).

Disk is generally cheap and comes in large sizes by default. You'll want to,
for performance reasons, separate that disk I/O as it is so unless this is a
SAN, you'll pretty much be limited to a 72GB (smallest I've seen lately)
drive size. That's larger than your Logs so I'll have to guess that you're
calculation for logs is based on SAN based storage that you can carve up
into smaller space.

Since Bharat has been kind and pointed out that size matters, you'll have to
dig a little more to work out the amount, but I see no reason to make it
smaller than the log drives. During normal operation you won't need it and
it'll look like it is being wasted.  When delivery stops and mail backs up,
and then dequeues, you'll wish you had used enough space to keep the server
operational until you could give it the attention it needed.

I'm sure I could wander on about this for hours, but I think you get the
idea. :)

Al