In 123 Flash Chat, there are three kinds of avatar sizes: big, small or none. Different avatar sizes can fit in the chat rooms of different scales.
Small Avatar
A chat room with high load and busy traffic are recommended to use the small avatar mode, (or even no avatar,) so to have more live content and more capacity in the chat lobby.

Large Avatar
If only around 50 users in each room with less busy traffic, you may choose the large avatar instead, so users’ personalities are more outstanding and the chat will be more colorful.

Configuration
Admin may configure a proper default avatar size for all chat users in this way:
Admin Panel -> Client Settings -> Themes -> Avatar Type, choose Large or Small Avatar, then save it.
(The regular chat users can define their personal avatar size in the Option Panel too.)

Related link:
The Upload/Webcam Module. Details