How do I make a line with Pen Tool with exact length?

Elias,

To elaborate on the exquisite Line Tool suggestion by Srishti, you have different options for building the boxes, depending on the need, Smart Guides being your friends (when used below they say anchor or intersection when you are within snapping distance).

For a(n initially) freeform build, you can Alt/OptionDrag the new copy line by the appropriate end Anchor Point to snap to the desired end Anchor Point on the original/former line, then switch to the Rotation Tool and click the point again to set it as the rotation centre, then ClickDrag by the other end as desired.

For a build with (mostly) specific angles, you can just Click the Artboard with the Line Tool for each line and set the angle (leaving the length as is), then ClickDrag the lines to snap.

Either way, to end up with the exact same length on the last two lines, I am afraid you will need to use circles as follows: create two circles with twice the length as width/height and centre them on the end Anchor Points to be joined with the two lines, then ClickDrag with the Line Tool to snap (either way).

And in the end, you need to join the lines into one closed path, of course.

I would suggest your using the Join Reasonably script in the free bundle by 佐藤浩之 (Satō Hiroyuki) here (or something similar):

http://shspage.com/aijs/en/