Mailbird scaling feature allows the user to adjust the size of the UI.
Here's what to do:
1) Go to the menu in the top left of the Mailbird window.
2) Select "Settings"
3) Then select "Appearance" tab.
As you can see we have 2 different levels of scaling, the app and the other.
The "Application scaling level"
Use the slider from left to right to adjust the level. It will scale the entire UI
The "Other scaling levels"
The individual scaling for email and apps will be still available and it will multiply the value by the whole scaling
Use the slider from left to right to adjust the text in your messages.
You can see the text in your messages increase or decrease when sliding the scaling bar from left to right.
You can also change the scaling of your apps. Do the same ways, use the slider from left to right to adjust the scaling level.
There is also scaling indicator in the top right of conversation. It will show up when you change the value or when you hover over the area. If you click it, it will reset the scaling to 100%. Please note this indicator is only for the email, not for the whole UI.
There is scaling indicator too in the bottom right of 3rd party apps. Same with the email scaling, this indicator not for the whole UI.