In version 18.104.22.168 the maximum size of the thumbnails is 240 (px?). When the thumbnail mode is selected on the viewer tab, the size can be controlled with the "Preview size" slider between 95 and 240. (480 or 720 size can not be selected, but it would be great)
If the host has multiple monitors (2, or even worse, 3), the maximum thumbnail size of 240 is for me too small. It's hard to see, if there are some error messages on the remote host, or something gone wrong, especially when the viewer runs on a 4K monitor.
I found the corresponding option in the config_4.xml file:
but changeing it to a bigger value in a text editor, causes the viewer to crash, when it's started. (The thumbnail frames are displayed larger, but the remote desktop image is not, the viewer hangs.)
I tought, that the thumbnail (avatar) remote images are the same way recieved and displayed, as the "Advanced hint window", that appears when hovering with the mouse. These hover hint windows can be pinned down, and resized to my desired bigger size.
If the thumbnails can't be made bigger, then i have another feature request. :)
I like those hint windows very much, i used to open and pin down 4-5 at one time, resize and arrange them, but the sizes and positions are lost, when they get closed.
Maybe they could be "remember" their positions and sizes (saved to the connection address book properties). Or even better: save multiple opened "Advanced hint window" arrangement as some sort of layout, then those different layouts could be opened. Like a "surveilance camera video wall", but for remote desktops.