These number represent the input shape you are passing to Linear.
Your activation shape right before the classifier is [batch_size, 256, 8, 8].
From the docs 4 of Linear we know, that the input shape to this layer should be [batch_size, *, input_features], where * means any number of additional dimensions.
I assume you used batch_size=64. Your input was therefore handled as [64*256*8, 8], which is a mismatch to the linear layer expecting 256*8*8=16384 features.
You would have to handle the shape depending on the layer. Usually if you change the layer type, e.g. from conv to linear, you want to flatten the activation. You can also reshape the activation so that you can pass it to a recurrent layer.