How to open new window of decompiler or resize the decompiler of Ghidra?

You can click on the top part of the desired window (which is blue in color when selected, and which has the title of the window written in it). You can then hold and drag it out of the screen.

At this point, it may appear weird that only this title bar is moving with your mouse, but try releasing the mouse button while this title bar is someone outside the ghidra screen, e.g., on another monitor even. You'll find that the window pops out and you can resize it, etc., as you please.

If on the other hand, you release it somewhere else within the ghidra screen, that repositions the window, possibly opening another tab to share the space with existing window(s) in that location.