jet helmets

In terms of coolness, jet helmets are ahead, but when it comes to safety, they are behind flip-up and full-face helmets.
Due to the open construction, your chin area is unprotected. In addition, jet helmets can slip more easily in the event of a fall whereas flip-up and full-face helmets offer reliable all-round protection.

As with all motorcycle helmets, the size of jet helmets is indicated in letters. These correspond to the head circumference.
The following measurements are hidden behind the individual abbreviations:

To find out what size you should order, place a measuring tape around your head at forehead height and read off the centimetre reading.

Only if the jet helmet fits perfectly it can fulfil its protective function in the event of a fall. The fit is correct when the helmet does not pinch uncomfortably, but is also not too loose.
If two fingers still fit between your head and the helmet after you put it on, it is too big.

This question cannot be answered with a definite yes or no.
Due to the half-shell shape, putting on a jet helmet is definitely easier for those who wear glasses than putting on a completely closed full-face helmet.
However, the helmet must not press uncomfortably on the temples of the glasses. The fit differs from model to model. However, you don't have to try out helmets at random and test them for comfort. You will usually find information on the pages of the online shops about whether the helmet offered is suitable for glasses or sunglasses.