The default is 10,000 which is suitable for images of 512by512 pixels. Reduce this for smaller images and increase it for larger ones. This is used to determine when to stop the minor itartions. This is normally determined from the psf and the number set here is only used if this cannot be determined. The default is zero.
Larger values mean more major iterations.
The default is zero. I have no experience on when to use this factor. Cleaning stops when the flux or iteration limit is reached. Determine the flux limit if we only select the maxNumPix biggest residuals. The maximum residual is the absolute maximum.