You have to specify a different (bigger) font in the UIPopupList's Font section. Popup list uses this font to create your labels.
I'm sorry; I find it difficult to believe that adding a second font to our project, giving it a larger default size, and directing the UIPopupList to use it, will give us additional options on UIPopupList..

Or is this supposed to somehow cause some sort of fallback behavior in the UIPopupList which will make it adopt the text size we specified in the UIPopupList's child UILabel?
I'm finding it more likely that either I am not understanding your answer or you are not understanding our question.. If I had to guess, based on your response, I'd guess that you thought we were trying to
achieve the result in the screenshot rather than
correct it.
For reference, here is a screenshot of that same UIPopupList, but under NGUI 2. Disregard the gray blocks I've added; as it's an old screenshot, I was not able to remove sensitive information beforehand.

Note that in the old version of NGUI, shown here, we were able to keep the text size of the UIPopupList's rows the same as the text size of the UILabel very easily, by setting textScale to the appropriate ratio. One of our developers who has looked at the code for the new UIPopupList says that, at first glance, it would seem that it be fully capable of drawing the entries at any desired size, but that it simply no longer exposes this functionality and is instead forcing it to use a ratio of 1 for whatever reason. Of course, he does not have a full understanding of how NGUI works in every aspect of what it does, like you do, so he could very easily be misinterpreting the implementation code. I only mention it in order to help communicate the issue we're trying to resolve. Are we able to keep the rows of a UIPopupList drawn to the same size as its UILabel (and if so, how?), or does NGUI no longer support UIPopupList with Unity 3.5.7?
I appreciate that you are trying to help us to get this resolved.