How small would the patch of sun be that delivers heat to the earth? The sun, being a ball, projects its EM radiation and light in all directions. We earthlings only receive a light from a portion of the sun, not all of it. What size is this patch of the sun's surface that sends light across 93m miles of space?
EDIT - There was a faulty premise in my question. We receive heat from all portions of the sun which are visible to us since EM propagation happens in all directions, not just most earth facing portion of the suns surface.  Thanks for the thorough comments! 
 A: We receive energy from the entire hemisphere of the sun which faces us (offset by a very small angle corresponding to the 8 minute travel time of light divided by the rotation rate of the sun). For proof, go outside on a sunny day and look at the sun. You see the entire disk, not just some tiny portion.
What you may be thinking of is the fraction of the sun's total output which reaches us, projected onto the sun's surface, and that can be calculated. The earth has a diameter of 12,756 km, and an orbital radius of about 150 million km. The radius of the sun is about 0.7 million km. So the apparent diameter of the earth projected on the surface of the sun is $$D = 12,756 \times{\frac{0.7 \text{ million}}{150 \text{ million}}} = 59.5 \text{ km}$$ The problem with this approach is that light from the sun is not exclusively emitted perpendicular to the surface of the sun, but is emitted in all directions.
EDIT - And I see from comments that my throwaway about proving that we see the entire sun rather than a patch has been criticized as circular. However, when I stated that "You see the entire disk", I assumed that it is well-known that stars and planets can be seen outside the visible disk. If we were only seeing part of the sun, the invisible portion would also block the light from those objects on the far side of the sun. This invisible portion (assuming a spherical sun) would form a dark ring around the sun. Since no such ring is observed, what we see comprises the entire sun, and the fact that we can see it as a disk of approximately uniform brightness demonstrates that light from the entire hemisphere reaches us. There is no circularity of reasoning involved.
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