For an input tensor of size $n_H \times n_W \times n_C$, the output size after applying a pooling layer with a square window of size $f$ and a stride of $s$ is:

$\lfloor(n_H - f)/s + 1\rfloor \times \lfloor(n_W - f)/s + 1\rfloor \times n_C$

Here, $n_H$, $n_W$, and $n_C$ are the height, width, and number of channels of the input layer. The number of output channels remains $n_C$ because pooling applies to each channel separately.