Hello All!
Generally, say on a sizable HD of about 20 GB, how big
should the /tmp and /boot (maybe 100mb) partitions be?
I have a pretty good idea about /usr,/swap and /var
Thanks!

me personally i have always made my boot partition 100mb regardless of HD size, as of tmp well again personally i have made mine 2gigs on a 60gig drive, and there is probably only about 100megs used ... so depending on how you do things, you could get away with a 1gig or even a 500meg partition ..
again it depends on how you do things, if you were in my shoes, you could get a way with a 250meg partition, cause i over guessed at the time when i made mine

In Fedora or RH it is recomended to have boot at least of 75 MB which you will be prompted with if you try to make it smaller. tmp can be mounted where / is mounted and that wouldn't make it any problems with the size.

The way I have it figured out is to have a /boot partition for booting purposes of 100 MB (just to be sure), / partition of any share of your harddisk you desire, but more than 10GB is for me just obscure and useless. Rather use your precious space for /home partition which is good to have when reinstalling and is where you keep your files (I mean big files too)).

Having a separate partition is much more useful than having a /tmp partition. /tmp is where the temp files are, which get deleted when they are not used or must be raplaced with newer files, so I wouldn't make a /tmp partition. I would however strongly recommend a /home partition, which will let you keep all your data when upgrading or reinstalling your Linux.

Peace!

Hope I've helped.