How do I get the SizeBox size in OnPaint?

You’re trying to get a slot the size box is in, but the size box is not in a size box slot. Here’s a tongue twister for ya!

Also, decide what you need: Size or Padding - two very different things.

Is this what you’re after?

How to get the SizeX and SizeY attribute values of vertical layout boxes in blueprints?

In this very example, the Canvas Slot will know the size of the Horizontal Box:

Or you could tick Size To Content and the Size Box' size will be used (if overriding), or get rid of the size box overrides and use the image size as desired size. The auto-sizing canvas slot would respect that.

But then again, no clue what the end goal here is.