How to change default font size when preview open microsoft excel document?
Enter the following command into the terminal. Quit and relaunch Preview.

defaults write com.apple.Preview NSSystemFontSize -int X


Change X to desired font size.

Source: http://www.macosxtips.co.uk/index_files/13-things-you-didnt-know-about-preview.php

Oh, and to Apple - ever heard of UI?!