Is something not working like it should? Let us know.
I admit, the first time I read this, I was confused so I tried to duplicate what you are seeing because I never look at the brush size dialog; it even took a some time to figure out. SO I made this test. I selected the Medium Basic 250 px brush, Zoom in to 25% brushed a stroke, then to 50% etc, all the way to 3200%. The brush stroke was the same size for all. Then I realized your confusion. The brush size stayed at 250 px size all that time as it should. The pixel brush size is independent of the zoom level and should remain the same. The only way to quickly change that brush size is with the bracket short cuts [ or ]. Or use the slider in the dialog.