When the initial Avast screen come up, instead of clicking on the maximize button move your cursor to one of the corners of the current screen. You should see a double sided arrow. When you see that arrow hold down you left mouse button and drag it to the size you want. You may need to go to several corners to get your desired display. Once satisfied with the size, close down the window and then bring it back up. See if that now holds to your desired size.