Gravitational-wave laser interferometer detectors use a pair of cylindrical vacuum tubes, placed perpendicular to each other, in which the partial pressure of hydrogen is required to be similar to 10(-9) mbar to obtain residual gas-phase noise of similar to 10(-25) Hz(-1/2). Because total strain sensitivity decreases proportionally to arm length, it is advantageous to use longer arm-length interferometers. The vacuum-tube diameters are set by the light-scattering requirements, and also by the possible need for additional optical cavities within the tube. Here, the authors investigate the balance between achievable vacuum and tube diameter for various pumping conditions. For a 5 km arm-length interferometer, they consider whether there is an advantage in reduced tube diameter. They also consider the impact of baffles. They show that the vacuum performance depends critically on the stainless-steel outgassing rate, which is limited by the tube conductance, and requires an outgassing rate < 5 X 10(-15) mbar 1 cm(-2) s(-1). It is shown that a smaller-diameter tube needs additional pumps, and that, in general, there is a reduced safety margin for practical design. The vacuum performance depends only weakly on the vacuum-pumping speed for practical pumps and is only weakly degraded by baffles. (c) 2007 American Vacuum Society.