In the top right corner, you have four icons :
- full screen mode
- minimize
- maximize
- close the window
Click on the first icon on the left. It will show the shared screen in full screen mode.
Below those four icons, you have a "Fit to screen" icon if all the presented content doesn't fit in your Lync window.
Actual size shows the content as it appears on presenter’s screen without scaling. This means if presented content is bigger than what can fit in your Lync window, scroll bars will be shown.
Fit to screen option on the other hand scales the image such that all the presented content can be seen without the need to scroll.