They not small at all pal. You can't judge a seedling by size for at least the 1st 14 days have passed then you'll know for sure if they are slow. To me, they look good.
Your RH is little low. I find that if I wet a bath towel and hang it from the top bar at the side of the tent. As it dries it'll bring her RH up 2 70+%.. but as I said. They look on track.
Enjoy the rest of your grow.