Well as far as increasing the size of bridges you need to look at them in the object editor under the Destructibles tab and the Bridges/Ramps folder. Look for the bridge you want to make bigger. Now increase the 'Editor - Maximum Scale' to something bigger than 1.00. Now you can increase the size of your bridge in the editor. (Keep in mind that with destructibles you have to increase the x,y,and z all together.) Now if you want to be able to use a giant bridge you need to look at the pathing map for the bridge. Chances are there wont be one that you can use. I would recommend removing the pathing. You can do this by finding the 'Pathing - Pathing Texture' field and setting it to NONE. Now you will have to make your own pathing texture around the bridge.
Set the bridge where you want it, and press 'P' this will make the pathing map show up. Pink means unwalkable, blue or nothing means walkable. So look at where your bridge is and make sure there is blue where you want to be able to walk (Under the bridge) and pink on the sides so units wont fall off the sides. Since the bridge is set to 'walkable' units will walk on the model as long as it has walkable ground underneath it. If you need to make somewhere walkable you can use invisible platforms. If you need to make ground unwalkable use pathing blockers. Both can be found in the doodad palette. (Invisible platforms under Bridges/Ramps, pathing blockers under Pathing Blockers.
Sorry for the book. Wanted to make it very clear. Hope this helps
