Hi Sebastian - there is no particular file size limit but there is a runtime memory size limit that MoI cannot use more than about 3.6GB of memory , or about 1.6 GB if you are on a 32-bit operating system. That's because MoI is a 32-bit program.

Runtime memory usage is a little different than just plain file size, because a lot of additional things like display mesh data are calculated for the runtime display, so there will usually be a quite a bit more memory consumed at runtime than just the file size.

679Mb is a quite large file though, and I'd generally recommend breaking something of that size up into a couple of different files, until your runtime memory consumption does not reach that approx 3.6GB limit - check the memory consumption of MoI.exe in Task Manager every once in a while to see if you are approaching that. A lot of things will generally stop to function if you actually hit the limit so it's best to leave a bit of cushion to make sure that you do not actually hit the limit and run into failed allocations.

Another thing that can help a lot is if you turn down the display mesh density to make coarser display meshes. That will make for a somewhat rougher looking display but it trims down runtime memory consumption by quite a bit. See this post about what settings to adjust for that:
http://moi3d.com/forum/index.php?webtag=MOI&msg=4643.4

- Michael