I´m a graphic designer who normally works with printing. I now have to convert an Illustrator CS5.1 file into dwg. When I do that the document size changes completely and most of the drawing is outside the format - I try to alter the document size afterwards, but than I have to save it in illustrator again and then the problem rises again when I export it to dwg.
DWG isn't a solid thing, it has been changed when new AutoCAD versions have appeared. Your Illustrator knows only few of existing DWG versions. As far as I know, Illustrator CS5.1 doesn't know newer DWG versions than 2004. It was the native format for AutoCAD versions which appeared in 2004...2006.
Do not expect right colors nor stroke widths when you open the exported DWG. Illustrator's layer structure can vanish as well as all effects.
When you export as DWG 2004 in Illustrator you should define how many inches or millimeters one DWG unit is going to present. When you open the DWG in another program you should define again how many inches or millimeters one DWG unit presents. I guess you have skipped one or both of those definitions.
Of course you can define in exporting also 1 DWG unit = xxxx pixels, picas or points, but I guess that's useless, because in common CAD software you must say how many inches or millimeters mean 1 unit. Try for example 1 unit = 1 millimeter in exporting and opening the DWG.