For the icons in the kmenu, edit ~/.kde/share/config/kickerrc.
(in case your new to linux, ~ represents your home directory. for me that makes the above /home/darkmatter/.kde/share/config/kickerrc.
You need to show hidden files (under "View" to see the .kde directory in Konqueror)
Find the menu section and change the size in "MenuEntryHeight=*", with the "*" being the icon size you want (I recommend setting the value to 16).
save your changes, then open a terminal and execute the following command
dcop kicker Panel restart
When the kicker restarts, you'll have nice, compact icons.
EDIT: piscikeeper beat me to it
As for themes, check kde-look, you may find some that are light enough for you.