By "orifice size" what Cathar is referring to is the restriction he locates immediately above the fins. It's cross sectional area should generally be lower than that of the block's flow path. It's purpose to to increase the average velocity at the point where the water initially hits the block, which wonder-of-wonders occurs directly over the die.
The balancing act here is between higher average velocity in the "jet" vs higher total flow volume (higher average velocity every except the "jet"). With a "small" orifice, you'll get a higher peak water velocity which will yield a higher convection coefficient directly beneath the orifice. You'll also get lower total flow volume, meaning a reduced convection coefficient outside of the "jet". Vice versa for a "large" orifice.
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