To estimate the width of narrow no-break space (NNBSP,
in a font, enter the name of the font family (as known to CSS)
below and use the Change button.
There is no check for the name actually referring to a font family in your system.
The NNBSP character is shown in large (1024 pixels) in
a field that is editable, so you can use this page for checking the width
of other characters (or strings) as well.
Results from tests on a Windows 10 system, in January 2021.
The properties of fonts may vary by version.