I am running Civil 3D 2012 it is an educational version. My question is when my dwg is over 100 megs It freezes up, Gives me a fatal error. I am wondering if Autodesk limits the size a dwg can get?  I have tried breaking the project into smaller pieces and then when I x-ref them it maxes out my dwg, corrupts the drawing and then will not recover....

There is no intentional limit but Civil 3D models can be quite RAM-hungry so make sure you have enough physical and virtual memory. 32-bit Windows versions are limited to 2GB (or 3GB) so use the 64-bit version of Windows (and Civil 3D) for such large DWG files.