Resizing that artboard to 200" would take it outside of the pasteboard bounds (perhaps you saw a message to this effect).
You have a couple of options:
1. Make a new document with an artboard the size you require.
2. Move that artboard to the far left corner of the pasteboard so that it has room to resize.
For reference, the maximum area in a normal Illustrator file is 227 inches square. If you require one larger than this, you must create a new document -- this will enable a 'large-sized' canvas/pasteboard (scaled up by a factor of 10).