You could use a little bit of trig/math to determine the length knowing the height and width values of the enclosing rectangle.
c = square root of (a squared + b squared)
Still annoying thing to do. I think that I have been using the wrong software, I mean, the reason I recently have been using flash for 2D design drawing, was because I am already familiar to flash because in the pass I used it for making some flash animations. I like how lines are attracted to the possible place where they are going to connect, and the easy way you can individually manipulate them, being this a good thing when need to fix some lines. I think that I am going to try Illustrator, never used it before. I hope there does not happens that problem with lines moving a little when very high resolution, because I realized that flash just cannot give me the accuracy that I need.
While I often relied on Flash as a drawing tool, for a variety of reasons where it proved useful to me (still do), its main purpose in life is not as a drawing tool but as an animation/dynamic interface tool. So you are likely making a better choice to pick drawing software instead of Flash if you are looking for precisely drawn elements.