do i can somehow lock the brush size to be the same despite the zoom ?
As far as I know, the Brush Size in Texture Paint Mode works with Pixels only.
If you are painting in the 3D Viewport it uses screen Pixels.
If you are painting in the Image Editor, it uses the Pixels of the Image.
As Omar mentioned, in Sculpt Mode you can set the Brush Size (Radius) to either View (as in Texture Paint), or Scene (zooming in or out, doesn't change the size of your Brush, relative to the Object you are Sculpting on...)
It would really be nice, if you had that choice in Texture Paint as well, but...we just have to work with what we got, for now...
thanks for the answers. its somewhat kinda sad . ill keep in mind an try without it then :)