The copper pipe should fit relatively tight around the glass tube (no more than 1-2 mm slack), and it must be longer than 12 cm (15 cm is okay). As the wavelength of the 2.4 GHz microwave radiation is 12 cm, no radiation can escape through a metal tube longer than that.
Minor correction, chief: if you don't want microwaves to leak to the outside, the diameter of the pipe should be smaller than 12 cm (or more precisely stated, the wavelength of the lowest mode possible in a cilindrical waveguide, which is IIRC a TE00). The length of the tube (or waveguide) can be any length.
And btw, for smaller diameters you still can have relatively much leakage to the outside, depending on the relation between wavelength of your radiation and the diameter of your hole...