When I was researching the A200, this is what I found on a very informational thread quoted below. It seems the actual plate that the skimmer sits on is probably 9"x15", but the exhaust extends beyond the base plate as well as the pump. See below info from the other thread, it is helpful to explain the question you are asking. If I were in your shoes, I would plan for 9.5"x19.5" to give yourself some space to work with as you don't want the pump pressing up right against your sump wall, that could cause a lot of vibration noise.
<a href=showthread.php?s=&postid=7118505#post7118505 target=_blank>Originally posted</a> by UCanDoIt
Well, I just received my A200 on Monday afternoon and my aquarium tech brought it over the same day and a lot of modifications needed to done to get this baby inside my tank stand and within my sump area.
Picture #3
You can see from this closeup that there's at least 2" that the Eheim sticks out beyond the elbow. From exhaust to elbow is 17", but to the end of the Eheim is about 19". Fins Reef has since revised their footprint measurement.
http://www.hsaquaristik.de/produktfotos/A-200-1260.jpg