As you might know, there are some shortcut keys to open Gmail’s new compose box in a new tab or new window, but before using them, you’ll need to enable shortcut keys from Gmail’s settings. Once you enable this option, you can press Shift + C to open the compose box in a new window, or press Ctrl + C to open it in a new tab.
Google has recently added this option to Gmail
However, the default is a small compose window that’s great for checking other emails and doing other tasks, but if you prefer a larger window, then you can now switch to full-screen compose box. Google has recently added this option to Gmail. When this option is enabled, the compose window is centered in your inbox and expands to fit on your screen. Also the formatting toolbar is enabled by default.
To enable full-screen compose box, click on the expand button (next to the close button) at the top right. It’s not exactly full-screen but it will open a maximized box. If you want to open the same compose box in a new window, then press Shift + click on the expand button.
If you prefer to set this full-screen option as default, then select “Default to full-screen” in the more options menu in the bottom right.
Note that this feature is slowly rolling out for everyone over the next couple of days. If you don’t see it now, then you’ll soon get it.