Menu -> File -> New

ò Width and Height ò

The width and height of your future image (up to 30000x30000).  In the listbox you may select most widespread sizes. "Square" checkbox allows to input only one size with copying content to another.

ò Seamless ò

ò Background ò

Defines background of an initial image.  If you choose "Blank" than starting background will become black. If you choose "File" than you may set an image file as initial background. Image sizes will be equals the size of an image file (up to 30000x30000).