I have defined a text bonding box (275px x 153 px for example).
Now I need to set this text to AUTO fit (resize both spacing and font size) this bounding box.
How can i do this in Photoshop?
Just use the Type Tool and click anywhere in your photoshop document. Type your text, paragraph etc. Once you're done, simply hit enter. Now (make sure your text layer is selected in the layers menu) then hit CTRL-T for transform (or select it from the menu: Edit/Transform/Scale) and click and hold any corner of the bounding box, hold the SHIFT ket to preserve the proportions of your text unless you want to stretch it voluntarily, and just adjust it the size you want!
As simple as that.
You cannot as you specifically describe it ... that the text will auto resize to best-fit into the box. Perhaps the reason why is that one can manually control both height and width percentage so how would PS automatically know what you want?