Dunno how many of you guys got Lightwave or 3DSMax, or something similar; though with the number of modders around here...
If you got it, it's even easier to figure out an exact length if you import a mesh of the craft in question, then scale it so that its in-program length is at the correct number of meters. Height and width are displayed right there for you, right down to the decimal.
Really helps out of you're terrible at basic arithmetic, like I am. D:
If you're wanting to take the measurement of an individual part, if I was doing it, I'd use the method I described above and then cut the parts you want out of the mesh. Isn't too hard - either boolean with a cube or copy the mesh, then edit it and delete all of the unwanted vertices and faces. When you're done, the new object will give you the exact dimensions.