Actually, I think it's pretty easy in MicroDEM. The author calls it "subsetting" instead of cropping. The subset icon you look for is a dotted-line square with an arrow in the bottom right corner, on the map toolbar, not on the main menu toolbar.
If you can't see the subset icon, you can also do it from the main menu: Modify - Map Area - Subset (graphical) & zoom.
I'm using MicroDEM 10.0 Alpha build 2006.6.16.4. This is from the Help file, sorry I can't paste the subset icon itself here.
Subset Map Area
... Icon on the map toolbar: Move to the NW corner of the area you want, hold down the left mouse button, and drag to the SE corner. The area selected will be redrawn, keeping the aspect ratio you selected and filling as much of the screen as possible.
This enables you to blow up the map, but cannot change the underlying resolution of the map data. You may begin to clearly see stair-stepping of the individual data points.
If you want to blow up a very small area of the screen, it is often best to do it in several steps. This will allow you to select the corners more closely. Graphical selection limits the coordinates you can select, and at smaller scales this limitation becomes more severe.