Just hit draw, line, type in the length (assuming drawing units is in mm), and hit enter. Make sure your in ortho mode (straight line mode) or have the correct angle.
use command "line" or "L"; input point "A" (or click on it), then input second point as "@10000<90". The 90 can be any angle you want, from 0 to 360 degrees.
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.