Q: Default Window Size Message app

Howdy kpasa,

Thanks for using the Apple Support Communities.

I understand you want to change the default size of the Messages window.  This can be accomplished by simply closing the window (clicking on the red X), and then re-opening Messages.  You will now see that your Messages window has relaunched into its previous size window.

OS X El Capitan: Window basics

Take care.

You might be using Split View.  

Exit Split View

1. Move the pointer to the top of the screen to show the window buttons, then click the full-screen button .
2. The other window remains in full-screen view. You can switch to it with Mission Control, or use a Multi-Touch gesture such as swiping left or right with four fingers on your trackpad.

Use two Mac apps side by side in Split View 

Hi kpasa,

Thank you for using the Apple Support Community. Due to the nature of your issue or question you may find more information by using another one of Apple's support resources - https://getsupport.apple.com/GetproductgroupList.action

Take care.