Use the Generate tool to quickly build out blank ad sizes. Simply create one ad size from scratch, and then use it as a source to build other like-sized ad dimensions.
Build out an ad size that most closely resembles the height and width of the blank canvas’ that need to be filled. For example, 300x250 will translate well to 300x600.
From any blank ad, click on the generate button at the top of of each size. Additionally, you can click on any ad to highlight it and click the File menu to find the generate option.
Select the recommended source size from the drop down menu.
Tweak any assets within the local ad size view to achieve pixel perfection.
Repeat. The more tweaks that happen in between each ad size creation will help the generator more accurately build out a blank size.