Did you use Plan>Export to SVG format? I haven't seen a limit there.
I just exported a huge plan to SVG. Result: a 74MB SVG file with 30591x30590 pixels. (The plan I used is about 306x306 meters.)
Inkscape had no problem opening it and that's where I got the pixel information from.

Which version of Sweet Home 3D did you use and on what system?