What is the definition of Cload for MOSFET drivers?
The data sheet will list the gate capacitance of a MOSFET. This is also one of the prime parameters in most parametric search engines online (for example, Digi-Key search.)

The symbol for gate capacitance is typically "Cgate" or "Cg" or "Ciss" (for "input capacitance.")

If you look at a typical MOSFET data sheet, you will see the gate capacitance listed. For small-signal discrete MOSFETs, it can be as low as 10 pF; for large power MOSFETs, it can be as high as 13000 pF (which is 13 nF.)

For example, the BUK9515-60E has an input capacitance of 2651 pF at 25V. The parametric search for this component can be found here:
http://www.digikey.com/scripts/DkSearch/dksus.dll?WT.z_header=search_go&amp;lang=en&amp;keywords=BUK9515-60E

The data sheet for this component can be found here:
http://www.nxp.com/documents/data_sheet/BUK9515-60E.pdf

The Ciss is listed on page 6/13 of that data sheet.