It is the --min-length parameter you are looking for. Was added to MACS last year and is not yet in the documentation on Github. If you call macs2 callpeak without further options you'll find

--min-length MINLEN   Minimum length of a peak. The DEFAULT value is the
                        predicted fragment size d. Note, if you set a value
                        smaller than the fragment size, it may have NO effect
                        on the result. For BROAD peak calling, try to set a
                        large value such as 500bps. You can also use
                        '--cutoff-analysis' option with default setting, and
                        check the column 'avelpeak' under different cutoff
                        values to decide a reasonable minlen value.