Click and drag out your canvas so it's about 900 x 700.
Choose the Rectangle shape. Then:
Go to the Select drop-down menu and choose "Transparent".
Make sure the Paint Bucket tool is selected.
You'll have something like this...
Select the 2 colored rectangles and Copy. Make sure your background color is set to white.
Paste and move to the right.
Paste again, move below (staggered). Make sure the lines are one-pixel thick, not thicker.
Paste again, move below so you have a group of 8 bricks.
Once you have a group of 8, select it and Copy.
Keep pasting and moving into place, staggering the colors.
Once your canvas is filled like this...
Use the rectangular Select tool and select the area so you have no white parts. Then Crop.