Hi Tucker,
please see the example screenshot in my original post, and here is another one. As you can see, in both of these cases the ad inserted into this placeholder is of a different size than what I selected for this placeholder. When I set the sized for other placeholders, for example in the sidebar, then every time I see an ad there it's of one of the sizes I set up. But with these two in-content placeholders it doesn't work - on desktop the ad is often narrower than 970px (the only width option I selected for desktop) and on mobile it's often wider than 300px (the only width option I selected for mobile and tablet). Whenever the placeholder happens to show the correct ad size on desktop, it's 970x250, which fits the width of the box neatly. And I want only this width of ads to show on desktop for this placeholder. On mobile, an example of the correct size would be 300x600, but sometimes I see ads there that are 359x717px.
So, my questions is how can I set these in-content placeholders in such a way that they only display ads that fit neatly within the width of the box, neigher crossing the borders of the box, nor being significantly narrower than the width of the box.