Get width of properties panel How do I get the width of the N Properties Panel in the 3D View using the Python API of Blender 2.8?
Area.width

The screen is made up of areas, each area has an x, y, width, height attribute. 

Python console test code: 

&gt;&gt;&gt; for a in C.screen.areas:
...     a.type, a.width
...     
('PROPERTIES', 346)
('CONSOLE', 931)
('VIEW_3D', 346)
('TEXT_EDITOR', 931)


Region.width

within each area there are regions. Commonly there are 'HEADER', 'UI', 'TOOLS' and 'WINDOW' regions.  For the 'VIEW_3D' region the N panel is region type  'UI'

&gt;&gt;&gt; for a in C.screen.areas:
...     if a.type == 'VIEW_3D':
...         break
...         
&gt;&gt;&gt; a.type
'VIEW_3D'

&gt;&gt;&gt; for r in a.regions:
...     r.type, r.width
...     
('HEADER', 931)
('TOOLS', 1)
('UI', 335)
('WINDOW', 931)


If you are writing UI code to go in the N panel then can get the width directly with

context.region.width


Related:

Is there a way to determine the left 3D view sidebar&#39;s current width with python?