tool-bar-max-label-size

tool-bar-max-label-size is a variable defined in `C source code'.
Its value is

14

Documentation:
Maximum number of characters a label can have to be shown.
The tool bar style must also show labels for this to have any effect, see
`tool-bar-style'.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.1 of Emacs.