Borders and background
Choose the page background color and the color and thickness of the borders around each cell (box) and around the entire heat map.
Size and shape
You can set the size of the entire heat map, in which case the size and shape of each cell will be determined by that size and how many rows and columns there are. Or set the size and shape of each cell, and the overall size is determined from that.
Optionally reverse the order of columns and/or rows. Or transpose rows and columns.