Is it possible with Bevel tool to type desired values directly, instead of applying it only by mouse movement?
You can bevel and use the Adjust last operation menu as Francois said, or you can while dragging the mouse to adjust the bevel width press:
A: to adjust the amount of width
M: to cycle through width type
P: to adjust the bevel profile
S: to adjust the number of segments
All these proprieties can be set using the numbpad.