In fitness clothes do you go up or down your size?
Wear what FITS. This may require try-ons in stores, as sizes are NO LONGER standardized, ever.
Depends. Some brands down, others up
If you know what size in regular clothing you are then you can buy fitness clothing in the same size and it should fit. I've been a medium (8-10) in regular clothing most of my life. Fitness clothing usually fits me in the same size. But you should check their size charts and return policy before you buy specially if you're buying on line.