When editing a heatmap the user can export an image of the heatmap to a bitmap file (PNG, JPG) that could be later used in a paper or in the web. The size depends on how you are viewing the heatmap on screen: if you have set the cell size to 3 pixel width and 5 pixel height this will be exactly the same in your bitmap file. Also, all the rows and columns selected to be visible will be printed in the bitmap file. Hide rows and columns via the contextual menu if you do not want to have them in your figure
Go to File --> Export --> Export heatmap as an image
Via the menu File --> Export --> Export scale as an image you are able to export the color scale of the currently viewed data.