How to change an unknown diameter of a hole to a specific size? I have a cube with several holes in it. Now I need to change the size of one hole to 6.34 mm diameter. What is the best way to do this?
Another way to achieve this is using the LoopTools addon's 'Circle tool'.

If the addon is not active on your system, go to the User Settings (CtrlAltU) > Addons > Mesh > LoopTools and activate it using the small checkbox on the right:



In Edit Mode select the loop or loops you want to resize, then either from the Tool Shelf > LoopTools or the Specials Menu (W > LoopTools) choose the 'Circle tool':



Toggle the "Radius" option on, then type in the radius you want (diameter / 2) for the hole: