i dont think there is a way to get the icons smaller. you could switch to higher resolution which makes the icons smaller. but there is some real good thing you could do to make more room for maya:

• get rid of the menu bar with ctrl-m - you have all that in your hotbox
• get rid of the annoying windows title bar disable option in window->setting/preferences->preferences->interface->show title bar in main window
• if you need the shelf, get rid if the shelf tabs arrow left of the shelfes -> disable shelf tabs - you can switch the shelfes with clicking the little tab thing above the arrow
• in hotbox on the east side on empty space -> disable all things you dont want to see - you can enable them later again using the hotboxes east side

hope some of this is of use to you to make more room to work in maya.