This is easily done, and there have been many examples posted in this forum.
Take a look at this Thread:
Newbie question: resize a window to central 1/3 of the screen
If that does not answer your question, here are many more topics on that subject:
https://www.google.com/search?q=site%3Aforum.keyboardmaestro.com%20Resize%20Window
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@catsvanbags, welcome to Keyboard Maestro (KM) and its Forum.
KM is one of the best Mac automation tools available, its Forum is one of the best and friendliest forums on the Internet. Whenever you reach a tough stumbling block trying to use KM, please feel free to post your question/problem here for help.
You will also find this helpful:
Tip: How Do I Get The Best Answer in the Shortest Time?
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Getting Started with Keyboard Maestro
- Read the Quick Start.
- This is essential to become familiar with KM terminology
- Do the tutorial (Help ➤ Tutorial) in the KM Editor.
- Gives you a live walkthrough of creating a macro in the KM Editor
- Review/Browse the Available Macro Actions
- For Help with an Action in your Macro, click on the gear icon
at the top right of the Action, and select "Help" - Search the Keyboard Maestro Wiki for detailed help.
- Start small, and grow your macros organically.
- Be prepared for some trial and error in the beginning.
- Make good use of this Keyboard Maestro Forum
- Checkout these Great Resources
- Search for existing macros
- Post your questions/issues if you get stuck
- Be sure to first read: Tip: How Do I Get The Best Answer in the Shortest Time?
For more help, see Getting Started with Keyboard Maestro and the Forum .