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With the exception of very particular cases it's very rare to have horizontal expansion derived from the ads, so usually there is no need to allocate any space based in the width.
And what to do when we don't know the height of the ads?
Some ad sizes like the 1x1 and fluid, are per default dynamic which makes it harder to set a specific size for allocation, as with the multi-size ads, checking the reports for the most common or bigger sizes and preemptively allocate it, it's an option, but the general recommendation is to keep this ads above the fold.
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