Create a new image by selecting New from the file menu or by using the Ctrl+N shortcut.
A New Blank Image dialog will allow you to set the image width, height, and color.
Size
The dimensions of the new image are controlled by the width and height fields.

Background Color
You can change the image color by clicking on the color button. This will bring up a color picker to let you choose a new background color for your image.
Alternatively, you can make the new image completely transparent by selecting the Transparent radio button.

Click the OK button at the bottom to create the new image.
Click the Cancel button at the bottom to close the dialog without creating the image.