FIR Filter matlab command
Matlab FIR-1 command designs FIR filters with at least two given parameters. P1: The order of the filter, P2: cutoff frequency, assuming a default lowpass filter. Order of the filter N determines a number of properties of the resulting filter such as its comptational complexity, transition bandwidth, passband and stop band ripples. In general large N gives better (closer to ideal) frequency response but worse computational load. Also order of filter being even or odd will have effect on zero-locations being at Nyquist frequency which might be important for some applications with high/low pass filters...

The second parameter, cutoff frequency, is basicaly determined by the application.