Since Windows Vista Mechanic runs in a Window by default.  It can be made to run full screen like previous versions of Windows but it involves dumbing down the video driver which will result in performance issues with other Windows applications and is therefore not recommended.

To change the size of the Mechanic window right click on the heading of the window and select Properties from the menu that appears.  Go to the Font tab and select the size you want.  The default Raster Font is limited in size while the other fonts can go very large.  You can enter your own sizes for these other fonts - for example if 28 is too small and 36 is too large you can manually enter 30 in the box at the top of the list.  To change the Font itself select it from the list half way down this window - the available sizes will change accordingly.  Click on OK and if asked if you want to change the properties of the icon answer yes.

If you have made the Mechanic window bigger than your physical screen scroll bars will appear on the right side and or bottom.  Once you reduce the font to a reasonable size the scroll bars will still be active but you can grab the bottom right corner of the Mechanic window and stretch it back out to the maximum size for that font and size combination.