Quoted diameter for R Dor is quite big, however it may not be a true record. With M.Lattanzi (Torino) and P.A.Whitelock and M.W.Feast (Cape Town) we have used HST to measure in interferometric mode with the onboard FGS the diameters of two O-rich Miras, R Leo and W Hya. At visual wavelengths (objects like these present a strong dependence of diameter on lambda) the two stars present marked elipticity and mean diameters of 74 mas (+/-2) and 84 mas (+/-2). Details and analysis in coming ApJ august I issue. Regards. Ulisse Munari, Padova-Asiago Observatories