Each character has the same width on air:
iiiiiiiiiiiiiiiiiiii
mmmmmmmmmmmmmmmmmmmm
Seeing is believing.
If the width allowed is 37 characters, there is room for 37 i's as well as 37 m's, but no more.
To prepare for monospaced text, select Setup/preferences/line break/break line at max character count. Each line may now hold X characters (default: 37).
See also: