Why is that? I use a Ultra wide monitor(21:9 29 in)... Might that be the problem? 2560x1080
Can i solve this somehow to avoid resizing the window every time?
I have no idea. The window size it's entirely outside our control. However, I don't understand why it gets resized when the PMDG is loaded. I don't think they made it intentionally, maybe there's something unusual in the airplane, like an invisible gauge or something like that, which is fooling P3D in calculating the wrong size.
Note that, the latest P3D Hotfix somewhat "fixed" this, by always adding a "Next..." menu entry when the menu size is too small to display all 10 entries. At least, you would know there *are* more entries to see.