frame-resize-pixelwise

frame-resize-pixelwise is a variable defined in `C source code'.
Its value is

nil

Documentation:
Non-nil means resize frames pixelwise.
If this option is nil, resizing a frame rounds its sizes to the frame's
current values of `frame-char-height' and `frame-char-width'. If this
is non-nil, no rounding occurs, hence frame sizes can increase/decrease
by one pixel.

With some window managers you may have to set this to non-nil in order
to set the size of a frame in pixels, to maximize frames or to make them
fullscreen. To resize your initial frame pixelwise, set this option to
a non-nil value in your init file.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.4 of Emacs.