Actually since I have mine open right next to me I can tell you. Internal dimensions measure just under 17" from left-to-right, and just under 14-1/4" front-to-back. Sorry European folks, you're going to have to convert that on your own. :p
I'm a Dell shop here and don't have any ATX motherboards handy, but I set one of Dell's proprietary BTX boards in... it looked kind of rediculous though, also the slots are on the wrong side, haha. 
Of course, you mean just under 43cm from left to right and just under 36.2cm front to back, where the formula is multiply the Inches by 2.54, as everyone should know. This system is used all over the World, but some people haven't caught up yet!
BTW, I was interested when I read in another recent post of yours that no one would want to have an Amiga meeting in your "Craig's List" type apartment, which is in a basement. I think you could benefit from getting someone from Craig's List, LETS (Local Exchange Trading Systems), or Parecon (Participatory Economics) to come round and tidy it up. My flat is 31.22 square metres, but since having someone from LETS come to tidy it up, I've felt for the first time that I could hold some kind of meeting here with about 3 or 4 people including me. Of course, you'll have to learn how big 31.22 square metres is and then measure your flat in metres to see how it compares. In the German soap opera Lindenstrasse which you can watch on
http://www.lindenstrasse.de , they've said that they think 40 square metres is too small to live in, or too small for a couple. In the main building at Lindenstrasse 3, where most of the main characters live, they have a "Hobby Keller" (Hobby Cellar) where they can do various activities such as band practice (i.e. a modern music group of 3 or 4 people), so it seems like that may be the kind of place you're living in. Perhaps you could find somewhere else in another building with a "Hobby Cellar". Here in London, there are often Residents' Halls attached to buildings or estates which can be used for various activities.
I don't think I could have an Amiga meeting here, though, because I haven't met any Amiga enthusiasts from London recently. I think it's more likely to be a meeting about home brewing or distilling and running moonshine!
