Window size is the amount of time over which a waveform is sampled, known as time record, expressed in samples. For example, a window size of 128 samples at a sample rate of 48 kHz. Equals a time record of 128 samples x 1/48000 seconds = 0,0027 seconds or 2,7 milliseconds.
So how does this determine frequency resolution?
An excellent video by Monty Montgomery @ xiph.org, clearing up some very common misconceptions about digital audio.