As wide as the narrow punctuation in a font, i.e. the advance width of the period or comma. HTML/XML named entity:  
com.typemytype.robofont.layerData
reference
anchors
components
baseGlyph
period
transformation
1
0
0
1
0
0
contours
lib
name
punctuationspace
unicodes
width
784
com.typemytype.robofont.mark
0.93
0.9
0.98
1.0
interface.gridadjust.original
{'width': 770.0, 'leftMargin': 0, 'rightMargin': 0}