You can do this in the 'Manage Grids' section in the admin.
The size of the grid is set in blocks (not pixels). Each block is a square of 100 pixels (10×10), so setting the size to 100 rows by 100 columns will give you 10,000 blocks which is exactly 1 million pixels.
Here are some example sizes:
– 200 rows and 100 columns will give you 2 million pixels.
– 50 rows and 50 columns will give you half a million pixels in a nice square
– 40 rows and 15 will give you a nice vertical banner with 60K pixels. (400×150 pixels)
The possibilities are endless, however please note that the script is limited to how much resources your server can take, and having grids over 2 million pixels would be infeasible because of the high resource requirements.