Say your input is ( Batch, input_channel, 64, 64)
Conv2D( input_channel, output_channel, 3, 1, 1) give you back the same size as the image.
output would be:
(Batch, output_channel, 64, 64)
Setting k_size, stride, padding, to 4, 2, 1 instead of 3, 1, 1 will give you ( Batch output_channel, 32, 32 ) instead.