Product review (5 stars): Worth a few extra bucks over the paper versions!
Compared to the paper version by Honeywell (HC26P), this is the pad to get.  It measures approximately 10x13x1.5.  I noticed an increase of humidity in the air after replacing the paper pad with this one.  There is also less water going straight to the drain, so it must be going into the air.

This pad has to be inserted into the plastic housing that is already in the humidifier.  Open the plastic housing, remove the old one, insert this one, and put the housing back.  Easy, no tools, what more can you want.

The added cost of approx $2 was well worth the quality increase.  The honeywell paper version says to replace twice in the winter.  This one should last all winter.