Goertzel algorithm magnitude
[1] It's a notch filter. It attenuates everything but the frequency component you're interested in. The output is summed up over some time to make the tone detection more reliable.
[2] There's no minimum block size. There's a requirement about the amount of time spent summing up the output. What you'll see is that the summed output over say 205 samples (at fs 8000kHz), grows much more if the input contains the frequency component you're interested in detecting.