It doesn't mather how long you line is for autocad, you can even draw a line of a lightyear long in it P-), just type in "l", hit enter, choose your basepoint, and type in "@1000000,0,0" or if the line is under a certain angle "@1000000<(angle)"
Then type in "dview", select the line, hit enter and orbit a bit (by clicking the cursor anywhere in the drawingfield and moving it and release the mouse), hit enter again to end the dview command and then zoom out to see the whole line.