auto-resize-tool-barsauto-resize-tool-bars is a variable defined in `
C source code'. Its value is
tDocumentation: Non-nil means automatically resize tool-bars. This dynamically changes the tool-bar's height to the minimum height that is needed to make all tool-bar items visible. If value is `grow-only', the tool-bar's height is only increased automatically; to decrease the tool-bar height, use M-x recenter.