  Context. Accurate long-baseline interferometric measurements require careful
calibration with reference stars. Small calibrators with high angular diameter
accuracy ensure the true visibility uncertainty to be dominated by the
measurement errors. Aims. We review some indirect methods for estimating
angular diameter, using various types of input data. Each diameter estimate,
obtained for the test-case calibrator star lambda Gru, is compared with the
value 2.71 mas found in the Bord\'e calibrator catalogue published in 2002.
Methods. Angular size estimations from spectral type, spectral index, in-band
magnitude, broadband photometry, and spectrophotometry give close estimates of
the angular diameter, with slightly variable uncertainties. Fits on photometry
and spectrophotometry need physical atmosphere models with "plausible" stellar
parameters. Angular diameter uncertainties were estimated by means of residual
bootstrapping confidence intervals. All numerical results and graphical outputs
presented in this paper were obtained using the routines developed under
PV-WAVE, which compose the modular software suite SPIDAST, created to calibrate
and interprete spectroscopic and interferometric measurements, particularly
those obtained with VLTI-AMBER. Results. The final angular diameter estimate
2.70 mas of lambda Gru, with 68% confidence interval 2.65-2.81 mas, is obtained
by fit of the MARCS model on the ISO-SWS 2.38-27.5 mum spectrum, with the
stellar parameters T_eff = 4 250 K, log(g) = 2.0, z = 0.0 dex, M = 1.0 M_sun,
and xi_turb = 2.0 km/s.
