Size of image adverts?

[Nihonto]

Thinking of adding some adverts, as simply as possible.
If I add them as images, and I do have some blocks in the sidebar and would like the adverts to be under those blocks, does anyone know the optimum width of the images/adverts?
And how do different screen sizes and mobile themes then handle the ads?
How to make the width fluid so it resizes with screen size? Is there a tutorial or a better plugin for this anywhere?
Thanks.

[Sonya*]

Hi,

you can see the most common banner sizes e. g. here https://support.google.com/google-ads/answer/7031480 I would recommend to use common sizes as the most companies/advertisers would have those formats. 

While uploading you image ads you can check Upload smaller versions? in AdminCP.

The first link gives you also the hint what sizes should be used on what devices. As an idea, you can use two versions: 300x200 on mobile and 300x600 on the desktop and tablets. I usually do not differ between desktop and tablets.

The images are resized automatically in IPS, though I would not want to resize automatically an ad format 970x90. It would be extreme small without proper mobile version if resized. In this case I would use 970x90 on desktop, 728x90 on tablets and 300x100 on mobile. 

There is no tutorial on what banner sizes would be better. It depends on the banner content and location on the page, so that you have to figure out yourself what banner in what size placed in special ad location looks good/bad on different devices.

[Nihonto]

Thank you Sonya. You have been most helpful.
Much appreciated.