There is a pdf by Hitman called "Tutorial for 100% realism manual targeting" available here:
http://www.subsim.com/radioroom/down...o=file&id=1647
In addition to being useful and informative in its own right, it also contains a page headed "Merchant Aspect Ratios" which has aspect ratios (length to mast height) for every merchant class in SH4. I believe the only use for length is to calculate the ship's aspect ratio, which is used in determining AoB. If you already have the aspect ratio, you can use it directly to determine AoB. The table is a single sheet (actually only half a page long) so you can easily keep it handy and refer to it while playing the game.
Of course, if you really want to know the ship's length for some reason, you can also use the aspect ratio. Multiply by the mast height (which is in the rec manual) to calculate the length.