This one shows holes and heat sink pads under the thermal vias. It's the bottom mask - areas of exposed copper. The thermal pads are 1cm square. I forget now the hole diameter, maybe 4mm but if there is a good standard size let me know. (Don't measure here as this includes a ring around the hole)
http://i.imgur.com/zMSgPvm.pngBkkCoins this article has a nice table of thermal resistance reduction for different vias arrays that can serve you as a reference their model uses a 10mm square die vs the 7mm Avalon chips has.
