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If you are changing system units make sure reset the scene and re import your drawings, otherwise these will not scale to the new units.
Also we cannot see your screenshot, upload it to imgBB or something and share the link
3d cargo here is a link
https://ibb.co/ygy00dS
and yes i have reset , i think the problem is not in system unit , because in feet and Cm both are givving the same result. think how can 12 feet wall so long ?
I think you may be correct about the system units not being an issue - it shouldnt be an issue unless you are merging 2 files with different units. Now regarding your issue, whats wrong with a 396 cm wall? Its hard to know what you are making do you have the drawings/plans available to look at?
yes let me show you . https://ibb.co/1G8WY82
https://ibb.co/yqDMk8d
its a small exterior house .
Looks like you have 2 problems. Problem 1: Check that your imported floorplan is to the correct scale, door widths are usually 80cm or 90cm. Problem 2: Where are you getting this information of the wall being 13foot? Which wall? The easiest way fix all your problems at once is simply measure one of the walls in CAD, then bring this into 3dsmax and make sure that same wall is the dimensions you just measured.