Epitaxial YBaCuO thin films on 73 mm diam. sapphire and 50 mm LaAlO3 wafers prepared for microwave applications were characterized by optical and electron microscopy as well as by X-ray diffraction in order to guide optimizing the film properties. The surface resistance R-s, measured at 8.5, 19 or 145 GHz, partially as a function of microwave field amplitude B-s, was taken as the key parameter. Typical results scaled quadratically in frequency to 0.25-0.75 m Omega at 10 GHz, 77 K, and low field levels. In case of sapphire substrates, a challenge for applications is microcracking of the films. However, cracks are less deleterious if ``dispersed'' by other heterogeneities such as a-oriented grains. Narrow microcracks should allow for tunnel currents and flux pinning. In case of LaAlO3, films sensitive to high microwave power exhibited some inplane rotational misorientation and a-oriented grains as well as inhomogeneous layering of these grains and of additional Cu-O planes within the film thickness, with possible influence on R-s. [References: 14]