Thank you for the image.
This area has default height 400px, you may change it in file modules\boonex\messenger\template\css\main.css
You need to find class .bx-messenger-table-wrapper div.bx-messenger-table-wrapper-rows:first-child .cell and replace 400px with your value.
Hello, ok, thanks for the tip.
Also I have another question regarding this block. Although the visibility option of the block is checked for "Not member", the block does not appear if you are not connected.
I do not know if this is a bug, or if technically it is not possible for a non-member to interfere in the messenger.
It's just a question I ask myself ...