It's not really possible to find a study that'd prove irrefutably that [insert font size here] is ideal or not ideal, because it depends so much on other parts of the design... as well as your target market, business model, etc. I would use different data in my response, but I'd also put it in the context of concepts they're already familiar with and give them a way to have some control without agreeing to use a huge ugly font.
I'd probably say something like this: "Some companies use 'black hat' SEO strategies to get their website to the first page of google results. Techniques like that can work to some extent, but in the long run they can have more drawbacks than advantages. Search engines constantly update their ranking strategies to prevent those tactics, and if your site is viewed as suspicious, they may end up removing you entirely. SEO is important, but part of good SEO is presenting content in such a way that it doesn't look spammy and marketing speak-y to the point that users begin to be skeptical about the quality of the product.
Similarly, if you put a phone number in an enormous font, it will draw the eye. However, the goal isn't just to get people to notice the existence of the phone number. Ultimately, we want visitors to dial the number. If we present our number in a way that's really different from our competitors, potential customers might consider it unattractive and unprofessional. For example, [more successful competitor]'s phone number is in a size 8 font. I think you're right that we should make ours bigger than theirs, but I want to make sure our customers will have a good reaction to the design. Size 10 or 12 seems like a good way to make ours bigger than [competitor] while continuing to ensure that our design looks better than theirs too."
Then I'd talk about the factors that you already know about - ones that do help people notice phone numbers (or any information, really) more than other parts of the website.
For example, http://www.useit.com/alertbox/scrolling-attention.html discusses the importance of putting information that you want people to see in full view rather than requiring them to scroll down because some users will never scroll down. http://www.useit.com/alertbox/horizontal-attention.html talks about where people tend to look first in terms of orientation (left, if they're in a country which reads left to right).
Many people don't like criticism, even if the extent of their research is "But I LIKE hot pink and I LIKE things that flash repeatedly!" Sometimes it's easier to dissuade people by proving that a different concept has data to back up its validity rather than trying to convince them that their own idea is not a good one. It often helps to refute them, present data backing up your position, and then quickly segue into something like "I'm glad you had the idea to make it bigger... I agree, and I'll enlarge it today to size 12. That reminds me, I've been meaning to ask you... how are other sites presenting phone numbers to get better search results? Should I be using parentheses or just dashes?" ;)