Thank you for your prompt replies !
Found something, but am scratching my head bald : can't really tell what the solution exactly is :
=>Parameters / Activate Large Sceen (16:9)
then =>Clip Attributs => Deform the clip (can't tell the exact term on the English screen)
then => Click right on the clip view => Adjust to the screen, or reinitialize deformation or conserve the proportion or original size. Here's the rub : I've the impression that what you see on the clip preview which seems to react to the option you clicked is not "remembered" when generating the output. So I just click crazy once or more in random order one one or more of these right-click options until the output gets right without understand what really happened.
And by the way, I feel that's it's absolutely silly to display "iphone","ipad", "ipod" options when what I really need to control is the proportion of the output for a larger "usefullness" than just boring "ipad","ipod","phone", idiotic limitations... I'm so pissed off I can't control an get a neat clean simple 16:9 output, or better : being able to define *MY* exact proportion or *ANY* trimmed size I want like 681px by 248px !!! Can be very useful to add into a Blender scene !
Thank you again.