the buff options are controlled by Discord unit frames / duf. There are a lot of options there so look through it all you'll find the options located on the left hand side. It takes some getting used to, but you'll figure it out.
Now check my casting bar how do i move & resize this? The Castbar settings in /duf is hided so i guess i must look somewhere else? Im Talking about that "Mind Flay" bar there