How do i make these values larger ?
The font size for the dimensions is the same size as the text in panels throughout Blender (ie, the fields in the right-hand properties panel in your screenshot. You can adjust the overall font size by adjusting the DPI setting in the System section of the User Preferences :
Note that adjusting the DPI will affect the whole Blender user interface, not just the labels in the 3d View.
While it may not be as simple to use, you can enable the MeasureIt addon which can add measurements at any font size you want. The MeasureIt addon was added to blender 2.78, it should be possible to manually install it in earlier versions.
By setting the text size in the MeasureIt Configuration, all items selected when segment is clicked will be given the same font size. Individual measurement display options can be adjusted in the properties region. Be sure to enable the display by clicking the Show/Hide button at the top.
You can find a video tutorial on using the MeasureIt addon here.