I'm trying to draw a line 200' long but my model space is only 50' wide - how do I change the limits ?AutoCAD 2015.TIA -

Turns on limits checking. When limits checking is on, you cannot enter points outside the grid limits. Because limits checking tests only points that you enter, portions of objects such as circles can extend outside the grid limits.
Command line -Limits (on)

Turn LIMITS off.  Nobody uses LIMITS any more.  Draw your line then do a Zoom > Extents and you'll see it.  AutoCAD will accommodate a line of practically infinite length.