Rigid Character always falls (Physics) (No Root Motion)

Hi Guys,

I have here a tiny little problem which drives me crazy. I wanna animate my char using No Root Motion because it makes my life easier.

I use this command to move the char the way he/she is looking at

 if (GUI.RepeatButton(new Rect(525, Screen.height - 150, 100, 50), "Forward") ){
         transform.position += transform.forward * Time.deltaTime * 5f;
     } 

My Problem is: When the Box Collider is too small, my character always falls when I move her. Using the Capsule Collider makes it even worse.

If the box collider is too large I cannot walk through narrow paths.

SO any idea, how to solve this newbie problem?

And no, I cannot turn off the Physics and I dont want to switch to Root Motion.

thank you guys