There is probably a way to do it, but unfortunately, I haven’t looked into it, as the image retrieval feature was developed before my fork. If I have some time, I’ll look into it. Feel free to open an issue on GitHub.
In the src/main.js file, you can manage the sizes for “SmallAvatar” and “LargeAvatar.”
However, you need to rebuild the plugin after making these changes. Be sure to follow the instructions in the “Build / contribute” section of the README on Github.
The plugin folder is located in ~/.config/Mailspring/packages/mailspring-avatars/ for Linux.