The report deals with measurements of electron bunch length using coherent backward transition radiation generated by a bunch in a gold target. Such bunches are used in novel compact laser- based acceleration. The aim is to compare efficiently non-polarized and polarized photons backward transition radiation for use in measurement of longitudinal size electron bunch. By calculation, it was shown that radiation with perpendicular polarization more effective than non-polarized radiation for sub-fs longitudinal size measuring. Calculation referred to in the report can be used for bunch diagnostics at SINBAD facility (DESY, Germany).