Are you referring to OS X or iOS? I've used both on my MacBook in the past and neither covered or caused the menu bar to not be visible. The same goes for the dock. If you're actually referring to the dock where some of the app icons are, right click on the dock, choose "Dock Preferences" and then uncheck the box next to "Automatically hide and show the Dock".
OS X, it is the menu bar at the top showing the app menu plus the bluetooth, volume, battery life, time, spotlight etc. that only shows when I hold the curser over it.
Thanks but by doing that (esc then resize window) you lose the tabs at the bottom of the excel sheet - i.e. worksheet tabs etc. Before this upgrade to Yosemite I always had full screen and the menu bar. Grrrr