Install Devilspie:
sudo apt-get update && sudo apt-get install devilspie
Devilspie is a non-gui utility that lets you make applications start in
specified workplaces, in specified sizes and placements, minimized or
maximized and much more based on simple config files.
Create a Devilspie configuration directory for the current user and then create a configuration file for devilspie, for example for gnome-terminal:
mkdir ~/.devilspie && echo gedit ~/.devilspie/gnome-terminal.ds
Add a desired contents, for example:
(if (is (application_name) "Terminal")
(begin
(geometry "800x400")
(center)
)
)
Then assign a custom keyboard shortcut in Cinnamon:
Go to main menu and search for app "Keyboard" and run it.
Choose the "Shortcuts" tab, choose Category "Custom Shortcuts" in the left and click "Add custom shortcut"

In the command window, enter "devilspie"

Click Add an in the Keyboard bindings, choose your desired shortcut, for example I chose Ctrl+Alt+D and close this window:

Now after pressing the shortcut, all open Terminal windows will change the size to desired size and will be centered.
Study the Devilspie documentation for other possibilities and usage.