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Yes, if you change your ad and it's properly labeled and you know where it's at on your site then the change will show up the way you anticipate it to. No need to fuss with the code. There should be a label in the source code that tells you what ad unit it corresponds to in your AdSense dashboard.
Default ad display preferences
Ad type (Content ads only): Text and image ads
Font family: Default font family
Font size: Large (or small or medium)
That setting applies to all of your ad units, all sizes, sitewide.
Posted by ken_b on March 28, 2013
I have 2 medium/large rectangle channels that consistently do with-in a few % of each other. The slight difference in size has never made a significant difference in performance. They run in the same spot on alternating pages. I set one for "Large Font" and left the other one at "Default Font".