How big is his head? Do you know the measurement? My 2 month old has a 17 inch head and I'm worried these will be way too big.

At his 9 month appointment it was 17 and 3/4 inches. They fit him comfortably they just tend to slide a little throughout the day so we have to watch them and adjust them.

Are these lenses polarized?

As far as I know they are. It was advertised as polarized

Do they provide 100% uva and uvb protection?

According to their website (http://banzworld.com/category/see-no-glare/) YES, they do protect against UVA and UVB radiation.

What is the difference between "baby" size and "0-2" size?

Hi,There is no difference between 'baby' and '0-2' sizes. It is just the way the original seller of this item has listed the item on AmazonThe glasses from the manufacturer are Baby Banz (0-2 years) and Kidz Banz (2-5 Years).Kindest RegardsPaul