TextStyle default fontOfSize: 21

Why do this piece of code provide a float object

| textf font  |
        font _ TextStyle default fontOfSize: 21.
        font pixelSize

:-) Dose someone know what 0.5 pixel means :-) (I joke sorry)

I have try to find pixelSize in StrikFontSet but I can only see it in
debug mode not with the browser? That why i don't want to touch it. I
don't know wether it is bind at run time.

Could somebody explain me how you can do it (aproximatively)?