avatar size

Hello there! I'm brand new to running a forum on my own and brand new to Xenforo. I was unable to figure out how to change the avatar size to the dimensions I need (250x500 preferably) and was told it could be changed with css. However, I'm not sure where to change the css or what to change it...

I would like to make the newsfeed more compact and orderly. I applied various changes through extra.css Currently the newsfeed uses a 48 x 48px avatar size. I would like to change this to 36 x 36px I tried setting this height for .event .avatar, .event .icon but that only reduces div height but...