Hi,
The theme has multiple safety checks that prevent the saving of values beyond the allowed limits (220 to 800px).
One possible workaround would be to export the theme settings, edit the value there (with care as the exported data is serialized) and then reimport the settings to force the custom value.
However, saving settings from the settings page ever again this will reset the value back to its default.
It would be simpler to apply the desired widths through custom CSS directly on the sidebar and main content elements.