As far as I know you can't change the font size in the address book within the Mail application. If the font is too small, use the real address book application (by clicking on the beige book with the @ on it in the dock). CMC-, brings up the preferences, where you can adjust the font size for this application (but not for the crippled one within Mail).
Sorry I can't provide any help, novetan. But to be honest, I never ever used the address book within Mail before I saw your question today. Instead of searching the address book, I just type the first few letters of the name in the address field and choose the address from the list that appears. Much faster that way.