The Gaussian filter does not have a sharp cut-off, but 2*pi*sigma is the wavelength at which the amplitude is multiplied by a factor of about 0.6 (more exactly, exp(-0.5)). So you might set sigma to 30/(2*pi) to suppress wavelengths over about 30 pixels.
hsize needs to be big enough that you don't truncate the filter too much. I use hsize = 2*ceil(2.6*sigma)+1 which gives roughly 1% truncation error on each dimension of the image, but you may wish to vary this depending on the trade-off between precision and compute time for your problem.
