I would not concern yourself with the size of a drip tray yet...that's the answer you're looking for after you ask the right questions.
First, draw it on paper first...tape two pieces lengthwise then find the center.
Second, the CENTER of the drawing will become the MIDDLE tap.
Third, set some glasses, mugs, and whatever you drink out of and place them on the paper. Then place tick marks (on the paper) at the base of the glasses. Be sure to allow a 1/2 - 1" space between the glasses.
Fourth, measure the distances from the center of all the glasses. Select the one that is the largest distance and use it to measure on both sides from the center. Use this template to drill your holes as they are now equally spaced.
Adding an inch or so past the left and right limits of the base of the glasses should give you an indicator of how long/wide a drip tray you'll need.
I may or may not have explained it well enough, but if you've ever drawn templates, etc, it's enough to get the point across. I hope it helped.
![Mug :mug: :mug:](https://cdn.homebrewtalk.com/smilies/sdrinking-100-154.gif)