Thanks Ambucias for your respond. I'll try to explain better.
Make a slide in Powerpoint on your Surface tablet, in the horizontal position, activate the slide show and the text becomes large and covers the total surface of the screen.
Now turn the tablet in the vertical position, the text becomes much smaller and covers only the centre of the screen.
So far so good?
Now what I would like to see, is that the text in the vertical positions has the same size and covers the whole screen, just as in the horizontal position, but in such a way that I can move from one slide to the other.
But maybe it is not possible.
Looking forward to your answer.
Sorry I write in english, french writing will be more difficult to me, but I'l try when really necessary.