For a big rectangle grid use MadJik's Grid/Checkerboard Maker.
Before you do, set your primary and secondary colours to what you want your rectangles' colours to be.
In the dialogue box, select "Checker Board". Uncheck the "Linked with Horizontal" option (unless you want squares rather than rectangles) and set the width with "Horizontal Grid Step" and the height with "Vertical Grid Step". The maximum width/height you can set is 300 pixels.
Should you want more than 300 pixels in any direction, use the Rectangle Select tool to select a row or column and then use the Move Selected Pixels tool to drag the row/column to the desired size.
Then, Select All (Ctrl+A) and again use the Move Selected Pixels tool to tilt/rotate the rectangles. If you need precise control over the angle of rotation, use the Layers > Rotate/Zoom (Ctrl+Shift+Z).