Shopper How to buy the right mask.?
RICHARD C best time to do, if possible, is to try the mask on from a dive shop first. the 'right' mask is one that creates a good seal on one's face... and that good seal is different for different people. then decide if you want a low volume mask (like the cressi panorama) or a higher volume mask that might offer the ability to change lens or have side windows or purge, etc.
if trying a particular model of a mask before hand is not possible at least try to find a similar mask from the same manufacturer to check the seal and the field of vision.