The effective genome size is the portion of the genome that is mappable. Large fractions of the genome are stretches of Ns that should be discarded. Also, if repetitive regions were not included in the mapping of reads, the effective genome size needs to be adjusted accordingly. Sizes are from the MACS2 website (--gsize)

This will override the auto detected tag size. Per default that option is deactivated: -1.0

Default=1e-5 (--pvalue)

The default 'auto' option calculates the maximum tags at the exact same location based on binomial distribution using 1e-5 as pvalue cutoff. The 'all' option keeps every tags. If an integer is given, at most this number of tags will be kept at the same location. The default is to keep one tag at the same location. (--keep-dup 1)

Buffer size for incrementally increasing internal array size to store reads alignment information. In most cases, you don't have to change this parameter. However, if there are large number of chromosomes/contigs/scaffolds in your alignment, it's recommended to specify a smaller buffer size in order to decrease memory usage (but it will take longer time to read alignment files).