Some helmets allow for the addition of extra padding for a good fit. A helmet should be snug, but not uncomfortable. If the helmet is not comfortable it's not for you. After finding the proper size, check that the chin strap fits snugly but comfortably under your chin. Always keep the chin strap fastened when skiing as it's the one thing keeping the helmet on your head in a crash.
Safety is the most important feature when looking at helmets.
The differences between these two certifications are significant. Both certifications have impact, stability, and penetration tests. A Snell certified helmet passes all of these tests at higher velocities than CE certified tests. Snell also does impact tests at more angles and with different objects. It's more difficult to find information on CE testing.