Mark,
Here are some scans of the proposed track layout. Each square represents 1 sq ft. and the baseboard is edged out in black. The squares along the back on the two scans add up to 16'. The sidings are just ideas and will be decided at the setting up stage.
First, the left hand (6' x 8') end:

The bridge is 5 ft long.
Now a version of the right hand (8' x 8') end:

The area to the left of the track on the end baseboard could be used for town with maybe some houses on the "plain" below.
Lastly, an alternative version of the right hand end:

Despite the bigger size, the scale is the same and the back run is just cut off short. This layout allows room for the town on the right hand side inside the track.
As regards the width of the run at the rear, I'll know exactly when I've got my latest DS order as I can then measure exactly the width available on the bridge supports. This should be in about 14 days' time, but if that's too late, I can make a good guess earlier.