You can change the size of the avatar by editing cometchat/themes/
THEMENAME/css/
cometchat.css file of your CometChat theme.

In comethat.css file, you need to change the height and width in .cometchat_userscontent
avatarimage and only height in .cometchat_userlist.

For example:

.cometchat_userscontentavatarimage {
height: 18px; /* Update height to your new height */
width: 18px; /* Update width to your new width */
}

and

.cometchat_userlist {
cursor: pointer;
height: 20px; /* Set this to 2px greater than the actual height */
line-height: 100%;
padding-top: 1px;
padding-bottom: 1px;
}