If you are dealing with constant size inputs, what I normally do is passing a dumpy input like dummpyInput = torch.zeros(someBatchSize, inputDim1, inputDim2) to the model.conv_layer() and then check the output shape so I can observe the output of the conv layer with my input size.
In order to do that efficiently you should gather the conv part and the ff part in  nn.Sequential class. So you can pass an input only to the conv part of the model.