Dependes on how you drew. 1st I assume you drew it in Mastercam solids. In the Extrude function you specify the distance to extrude. Go to the solids History tree, click on the extrude of the table and change the distance.
If you drew the rectangle from the top and extruded down, do it like Matt said. It's better to draw it from the side and extrude it out. Then you can make it any length, any time.
Go To My Web Site. There are some free training videos that show solid modeling. They might help you.
Mike Mattera
Tips For Manufacturing Training CD's, DVD's for Mastercam, SolidWorks, Inventor, G-Code Training & More
http://www.tipsforcadcam.com