  Type II Cepheids are old pulsating stars that can be used to trace the
distribution of an old stellar population and to measure distances to globular
clusters and galaxies within several megaparsecs. One method that can be used
to measure the distances of Type II Cepheids relies on period-luminosity
relations, which are quite widely explored in the literature. The
semi-geometrical Baade-Wesselink technique is another method that allows
distances of radially pulsating stars, such as Type II Cepheids, to be measured
if the so-called projection factor is known. Using the surface
brightness-colour relation version of the Baade-Wesselink technique, we
determined the projection factors and radii of eight nearby BL Her type stars.
We adopted accurate distances of target stars from Gaia Data Release 3. Time
series photometry in the V and K bands have been collected with two telescopes
located at the Rolf Chini Cerro Murphy Observatory, while spectroscopic data
have been obtained with instruments hosted by the European Southern
Observatory. The measured projection factors for the stars with good quality
data are in the range between 1.21 and 1.36. The typical uncertainty of
projection factors is 0.1. The mean value is 1.330$\pm$0.058, which gives the
uncertainty of $\sim$4%. The main sources of uncertainty on the p-factors are
statistical errors of the Baade-Wesselink fit and parallax. In the case of
radii, the biggest contribution to the error budget comes from the K band
photometry systematic uncertainty and parallax. The determined radii allowed us
to construct the period-radius relation for BL Her stars. Our period-radius
relation is in good agreement with the previous empirical calibration, while
two theoretical calibrations found in the literature agree with our relation
within 2$\sigma$. We also confirm that BL Her and RR Lyr stars obey an apparent
common period-radius relation.
