The Marvin desktop app should remember the size and location of the Marvin window, as well as the size and location of the Top of Mind window. If you find that the position and size of these windows isn't being saved, then something has gone wrong!
First, open the App Dir (on windows and linux, press alt to display the help menu):
Here you will find two files called window-state.json and topOfMind-state.json. First exit Marvin, then please send the contents of these files to support, and finally delete them. If this doesn't resolve the issue, please contact support.
On MacOS you can manually open this directory by opening a new Finder window, pressing Cmd+Shift+G and pasting in: