Both Canon SX20 IS and Canon SX510 HS have 1/2.3-inch sized 12.0 MP resolution sensors so sensor size and resolution is not a differentiator between these two cameras.
Below you can see the SX20 IS and SX510 HS sensor size comparison.
Sensor Size and Resolution Comparison image of Canon SX20 IS and Canon SX510 HS Cameras
Canon SX20 IS and Canon SX510 HS have the same sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture. On the other hand, since Canon SX20 IS has 32% larger pixel area compared to Canon SX510 HS, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.
Sensor size sets the circle of confusion, so it drives how much depth of field each
of these bodies gives you at a given aperture. You can calculate it exactly for the
Canon SX20 IS
or the
Canon SX510 HS ,
including hyperfocal distance.
