All the hockey players are not very tall.

The height range is from 5'7" to 6'3". Hockey needs speed and skill, even body checks are at very fast speed. So being quick and flexible is very important.

If your 6 years old is able to skate somehow (forward, backward and stop), then he is ready to play hockey.A few kids I saw who never skated at beginning, half year later can skate so well, like little pros.

Also to your point of safety, minor hockey level is not an issue at all. When players turn to 12 years old, that is when body checks get invloved.