I'm guessing you have anchored them correctly but for the size you should be using the viewport_size as a multiplier instead of hardcoding it.
let's say you development viewport size 100x200. In this size,your control has a size of 20x20. You don't put 20x20 in your control size, but rather you set it via code during _ready().
rect_size.x = get_viewport().size.x / 5
rect_size.y = get_viewport().size.y / 10
Now when you resize your viewport to 200x400, your control will automatically resize to 40x40 but stay anchored at the right place.