"Even if small is less beautiful, it is more efficient" - Graeme McCorkell
One of the great things about direct marketers is they test all sorts of stuff. They test offers, lists, formats, creative, media and lots more. So we have loads of learnings available.
Graeme McCorkell is a highly regarded direct marketing professional and author. The above quote refers to press advertising sizes. Tests show that smaller ads are more (cost) efficient than larger ads.
Sure, you will get higher responses if you make the ad bigger, but the additional responses will not be in proportion to the size increase. So for example, if you double the size of your ad, you won't double the number of responses.
A good rule of thumb is to expect 40% more responses if you double your ad size. But good to test for yourself.