Q: how can i change the size of the picture i have copied and paste

If this question is about TextEdit, I don't think you can do it in TextEdit, after pasting the image.  TextEdit is meant to be a basic text editing program.

You will have to open the image file in an image editing application.  You can use Preview for doing this, or something more fancy.  If it's Preview, just open the image file.  From the menu bar, under Tools, select Adjust Size.  Make the image the desired size.  Then, select (outline) the portion of the image you want in the Preview document window.  Copy it, and paste it into the TextEdit document window.