This requires changing CSS. Here’s a basic example to start with that increases the size from 45px to 60px.
.topic-avatar {
width: 60px; // increases the width of the avatar container
img.avatar { // increases the size of the avatar
width: 60px;
height: 60px;
}
}
As the above post mentioned, there are topics that cover this specific question in various ways here on Meta. It might be more productive to search before asking.
https://meta.discourse.org/t/how-to-change-topic-avatar-size/20689