What height should the base cabinets be if it's going to be used as a workbench?

Your base cabinets should be 35-inches tall if you want to use them as a workbench.

How far do the drawer boxes come out?

All of our drawers come out the full depth

What to do if the cabinet shelves are loose and come out of the shelf pins?

A good solution to this that will help are door bumper stickers. (read more)

What is the leg room size I should have between my base cabinets for a sitting area?

The leg room should be a minimum of 24". This allows plenty of room to move your legs around while sitting down.

How much weight does a shelf hold?

Each shelf holds up to 50 lbs. Unless you are storing extremely heavy items, 50lbs per shelf is typically sufficient.