'pacsysclean -o -dt' does work, but neither -i nor -s seems to be doing anything.

I sent a patch to fix the missing hyphen. You need to use the -o option with pacman options not package names. If you want to know how much space a certain package takes just use 'pacman -Qi'. Not all pacman query options affect the output, it's probably only usefull for -dt and -et.

I'm honestly at a loss what is the use case for pacsysclean. I've read the intro http://mailman.archlinux.org/pipermail/pacman-dev/2011-November/014754.html and have nothing against keeping this script, but ...
If I run 'pacman -Qdt' and I want to keep some of these packages, I change the install reason to 'explicitly installed'. All the others get removed, doesn't matter if big or small. Removing them with 'pacman -Rns' may free even more space if they have large dependencies (which won't be shown by 'pacman -Qdt').

1. 'expac "%m %n" $(pacman -Qdt) | sort -g | column -t' (and similar queries) is much faster but doesn't do KiB - MiB - GiB conversion. Yes, expac is an extra package.
2. 'pacsysclean -o pkgname' is IMHO actually useful, but you would have to change the "man page" a bit.
3. What are "custom pacman query options"? I guess you mean "that's the way you can alter the default 'pacsysclean' behaviour", but 'custom' feels out of place + as you said, there are few options that make sense.
4. "pacsysclean - Sort installed packages by decreasing installed size." - I would say it sorts by _increasing_size.

With regard to point 1. from my previous comment: I was wrong, expac does have '--humansize' switch, so 'expac -H M "%m\t%n" | sort -g' gives pretty similar output to 'pacsysclean'.