Today is day 10 after seeds showed from rockwools....

Yes, they do seem a bit small for ten days old. Not sure why, but as a first step I would be removing the plastic cover ASAP as your seedlings need good air exchange to grow properly and particularly the roots need oxygen, something that your cover may be limiting. Cannabis seedlings or cannabis in general does not need excessive artificially boosted humidity levels, it is not a rain forest plant. Most indicas come from the middle east region from countries like Pakistan and Afghanistan, who's climate is far from humid. Sativas, although they come from equatorial regions, which conveys images of lush tropics, are however grown during the "dry" season and not during the rainy and humid "wet" season. The only time cannabis needs high humidity is when cuttings have been taken, as they have no roots to be able to hydrate themselves. Another problem with covers, is that they can create mini ovens with much higher temps inside the "humidity dome" possibly stressing your seedlings. Combining the two issues of extra heat and low air exchange could basically be suffocating your plants and limiting their growth. Give them fresh air with normal humidity and I am sure your seedlings will start growing much faster. The "humidity dome" is also a prime environment for moulds and fungus to thrive in. Hope this helps,.......... Organoman.