it's probably a setting I've not enabled, but don't seem able to change the icon size of buffs/debuffs etc, all the changes are greyed out. Can anyone tell me what I'm doing wrong? see screenshot https://simonparker.tinytake.com/msc/ODU3NjA2OF8yMTkyMTM3NQ

You have to select Custom in the Icon Style drop down menu. Afterwards you can change settings there. Presets Square and Wide cannot be adjusted.
It's a bit complicated, but I could come up with no better solution in the options :)