In the new Vegas 19 in the PanCrop screen, I am not able to change screen size or to unlock the Lock Aspect Ratio. I can in Vegas 18. Need to fix problem for V19.
VEGAS Pro 19 isn't exactly "new" as it's a year old and soon to be replace by 20. For the "Lock Aspect Ratio", just click on the button. You won't see anything happen until you move the mouse away from the button. It's a little confusing in that you can click it all day and won't see a change unless you move the mouse away from the button. So go to the button, click it once, and then move away from the button.
Not sure what you mean by "Change the screen size", I can put my mouse in the lower right-hand corner of the window and resize however I want. Now, if it's docked that would prevent you from resizing it. By default, it is not docked.