Thanks @Pablo for the report,
There is an issue with Mattermost Desktop 3.4 where increasing or decreasing font doesn't work properly.
This should be mostly fixed with the next release, so that after you click at least once somewhere in the app, it'll work properly. A complete fix for it is still in the works.
In the meantime, if you reset to default text size, you'll be able to see the chat window at full screen. To set the default text size, you can either:
1) Press CTRL + 0 on your keyboard
2) Go to View > Actual Size in the menu bar at the top of the Windows app
Let me know if that helps resolve the issue?