Buffer-menu-buffer+size-width is a variable defined in `buff-menu.el'.
Its value is

  • This variable is obsolete since 24.3;
    use `Buffer-menu-name-width' and `Buffer-menu-size-width' instead.

Combined width of buffer name and size columns in Buffer Menu.
If nil, use `Buffer-menu-name-width' and `Buffer-menu-size-width'.

If non-nil, the value of `Buffer-menu-name-width' is overridden;
the name column is assigned width `Buffer-menu-buffer+size-width'
minus `Buffer-menu-size-width'. This use is deprecated.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.3 of Emacs.