Continuing the discussion from Background Pic Template to Use:
A while back @Lisa_Scumpieru shared her template and tutorial for creating great looking banners for your formatives.
Adding to that, there is a cool Page Ruler extension on Chrome that allows you to measure the pixels of things on a page. Instead of having to guess the sizes of your embeds or anything you wish to modify you can simply preview your formative, measure the size of want and change the height and width on your embed code. (tutorial)