The problem with resizable GUI is that it will only resize the outer GUI frame, the window. It won't resize the buttons and text so that's pointless as it'll only bring more visual/graphical issues than anything good. Unless they have an on-the-fly resize engine that will be able to resize buttons and text as well with a locked aspect ratio, so you can only resize window diagonally, keeping proper aspect ratio of buttons (so they don't get skewed).
Technically I'd keep it at 1000x600, but there are still netbooks with 1024x600 resolution that are very much capable of running Windows 10 and avast! Antivirus and they won't be able to use it because avast! GUI would cover the taskbar, filling entire screen. That's why I was aiming at vertical GUI size below 600 pixels.
Besides, look at competition like BitDefender and Kaspersky. I think their GUI's are well below 1000x600 and they work perfectly fine. I'm talking just the GUI size, nothing else.