Somehow I’ve doubled the size of all smplayer windows.
I can’t change it back, because I can’t see the lower third of the preference pages, and so can’t hit the apply button. I can’t change anything at all now.
I’m not able to change the size of the window in any way, either.
Hold the Meta key on your keyboard to click and drag with the mouse. (It should work on any “surface” of most applications.)
While you’re holding down Meta, you can click in the middle of the SMPlayer window and “drag” it upwards to be able to access the lower part of the application’s window.
(The “Meta” key is also known as the “Super” key, which is usually denoted by the BEAAAAUTIFUL “Windows” logo.)
In addition to what @winnie said, holding down Meta and using the right-hand mouse button allows you to resize a window without having to position the cursor on the window border.
However, I suspect your problem may have been caused by scaling. Without knowing for certain what you did, it is going to be very difficult to help you.
Click and drag the window “upwards” while using the Meta key, so that you can reveal the bottom part of the Preferences window to reveal the Apply button.