buffers-menu-buffer-name-length

buffers-menu-buffer-name-length is a variable defined in `menu-bar.el'.
Its value is

30

Documentation:
Maximum length of the buffer name on the Buffers menu.
If this is a number, then buffer names are truncated to this length.
If this is nil, then buffer names are shown in full.
A large number or nil makes the menu too wide.

You can customize this variable.