MyBB Community Forums

Go to Admin CP -> Templates & Style -> Themes -> Theme Name -> global.css click Edit Stylesheet: Advanced Mode

Then find:

.post .post_author div.author_avatar {
	float: left;
	margin-right: 3px;
}

.post.classic .post_author div.author_avatar {
	float: none;
	text-align: center;
	margin-bottom: 8px;
}

.post .post_author div.author_avatar img {
	padding: 5px;
	position: relative;
	background-color: rgb(250, 251, 252);
	padding: 2px;
	border: 1px solid rgb(213, 213, 213);
	-webkit-box-shadow: 0px 2px 2px rgba(0,0,0,0.1);
	-moz-box-shadow: 0px 2px 2px rgba(0,0,0,0.1);
	-khtml-box-shadow: 0px 2px 2px rgba(0,0,0,0.1);
	box-shadow: 0px 2px 2px rgba(0,0,0,0.1);
}

And replace those three css blocks with

.post .post_author div.author_avatar {
	float: left;
	margin-right: 3px;
	height:100px;
	width:100px;
}

.post.classic .post_author div.author_avatar {
	float: none;
	height:100px;
	width:100px;
	text-align: center;
	margin-bottom: 8px;
}

.post .post_author div.author_avatar img {
	position: relative;
	height:100px;
	width:100px;
}

Max sure to change height: 100px; and width: 100px; to the size that you want.