You can change the max memory used by Sweet Home 3D only on the installer version, by changing the value in the -Xmx Java option:
- under Linux, edit SweetHome3D script and update the 1024 value in the option -Xmx1024m,
- under Mac OS X, right click on Sweet Home 3D application icon, select Show Package Contents, open Contents folder, edit Info.plist file by dragging its icon on TextEdit application, and update the 1024 value in the text <string>-Xmx1024m</string>,
- under Windows, create a text file named SweetHome3D.l4j.ini in the same directory as SweetHome3D.exe, and write the max memory Java option in that file, for example: for 2 GB, then relaunch Sweet Home 3D.

On all systems, 1024m means 1024 MegaBytes.