When you select the Extrude button the middle of the tool panel adds a few options, but none for length of extrusion (this is 3D not CAD). Fortunately, altering the length of an extrusion is somewhat painless.
- Resize the object.
Select all of the verticees at one end of the extrusion and move them.
Dont forget the View menu lets you place a bitmap on the backround for reference.
Art