what you (Douglas) could do is buy a bigger drive (36GB if you have 18's) and plop that in as a spare, then leave only one 18GB spare and the 36GB spare and it should build on the 18GB first (if the rest of your disks are 18 of course) then if you have another loss, it'll use the 36GB.
Unless the failed disk is the parity, in which case the 36gb will be used. Putting only 1 of a larger disk is always a dangerous thing, as you're essentially running w/o a spare.
Further, if you rebuild a 18 onto a 36, you _don't_ get a bigger volume -- you get a more expensive 18gb disk (which you'd probably have to relabel to get the filer to use all 36gb of again).
Some features to support rolling upgrades of disks would be damn smooth though. (proactively fail a disk onto a bigger one, which grows the volume and rebuilds).. Makes an upgrade path to $BIGDISK[$#BIGDISK+1] quite attractive.
..kg..