according to Toshiba 90nm "logic densities up to 400,000 gates/mm2 or more and low-k dielectric"
http://www.toshiba.com/taec/Catalog/Line.do?familyid=1&lineid=7231

Helion Fast Hash IP is SHA-512 @ 2400 Mbps using 41500 gates

2400 Mbps / 512 = 4.688 MHashs/s per unit; Bitcoin is 256+256

20mm x 20 mm = 400 mm2 x 400k gates = 160,000k gates / 41.5k gates = 3855 units

3855 units x 4.688 MHashs/s = 18.072 GHashs/s per chip @ 200Mhz