I am an in-house designer and we have a lot of custom ad sizes. There are times I need to design a web ad for 10 different layouts. Currently, I have a master PSD with all the different sized artboards for the job. However it can be time consuming remaking adjusting the layout. Is there a better way to generate these ads at custom sizes? Look to achieve something like this Online Banner Generator, Create multiple banners at once
If the sizes of the layouts are proportional you can export from Photoshop to the different sizes. If the proportion of the sizes are different one option is work with linked files: Work with Smart Objects in Photoshop
I would think with all the different web devices around these days and the various display sizes and resolutions you would need many different layout and content to be approbate for the different device displays. Its not just a matter of scaling you need your multiple layout and the content will vary between them as will the banners. If I were you I would use google searches to see what I could find that may be helpful for what you want to do. You know what you would like to do your best qualified to do the searching. Consulting a professionl web design consultant may be the best starting place. Help you know what to search for.