I'm not 100% sure about going with the dirtbike tires either. I'm gonna try it with the knobbies and see how she goes, but if I'm not comfortable on it, I'm going to switch out the tires for something more ridable. I'm also steering more towards a more solid hardtail. The more I mock it up, I'm seeing that I'm gonna be squished trying to sit on it.
I agree with Joe, I'm building this, I want it to fit me. I want to be comfortable.