your design is not very well done. If you intent to machine it, you need to polish the 3D a lot, you have features that are close to imposible to machine.
I'd advise you to divide the part into 2 parts that are easier to manufacture separately and then assemble.

I answered a similar question in a different forum:

There are two solutions within CATIA to find the smallest rough stock size:
1. the Rough Stock command, and
2. the Measure Inertia command.

The Rough Stock command is available in the GSD workbench, but only when you have one of the NC Manufacturing licenses.

The Measure Inertia command requires that you customize the options and include the Principal Axes, and the three dimensions will be displayed at the bottom of the Measure branch of the tree (BBLx, BBLy, BBLz).

Here's a post I found when searching for "rough stock": http://www.eng-tips.com/viewthread.cfm?qid=128636

I recall several users posted that both of these commands were based on the Principal Axes, which is not always the smallest bounding box. So, macros were suggested and written. (A previous client of mine, wrote a macro that added 1/2" of stock all around the minimum stock size.)