When we artists have finished a painting we naturally want it all to be seen and the thought of cutting a piece out is very unwelcome. However a smaller part of the painting may say what you want to say more powefully than the full sheet.
It is worth considering and we can now easily crop without resorting to the scissors or bits of mount board. Most of us have access to a photo editing package online such as Photoshop or an app and it is here we can try out all manner of crops without touching the original. It's reversible and it is great fun!
Before you decide to crop ask yourself the following questions...
Does my painting speak more loudly with the crop? Or am I seeking a pretty part that might look good in a frame?
Be true to your original intent. That pretty part is probably like thousands of others out there, attractive but readily forgettable.
What can I learn from this for the next painting?
Cropping, when it works, usually makes a painting more simple, more focussed and therefore more powerful. That tells you a lot about your approach... concentrate on the essential.