I was playing around with the avatar dimension settings in the ACP but couldn't find the default HTML in the style sheets or layout templates. I inspected the page and saw that the default dimensions are set to 96x96.
Is there an easy way to tweak this in the mini profile without completely customizing the section?
Try something like this:
.reply .reply__row > div, .reply .reply__main .reply__user {
width: 300px;
}