Quoted from Mk1Mod0:
First, of course, I did a dozen or so tests to find the maximum drill bit width allowable while maintaining grip of the screw and structural integrity of the part. Any good fabricator knows you always test it first. You'll know the hole is too small when...
But for future reference, there is a drill size chart for screws sizes available - although not sure how well this translates for use in plastic: http://www.wlfuller.com/html/wood_screw_chart.html
Or this one if using machine screws: http://www.boltdepot.com/fastener-information/us-tap-drill-size.aspx