please, could you print me your input shape.
I tried, Here is code
class Net(nn.Module):

    def __init__(self):
        super(Net, self).__init__()
        # 1 input image channel, 6 output channels, 3x3 square convolution
        # kernel
        self.conv1 = nn.Conv2d(3, 6, 3)
        self.conv2 = nn.Conv2d(6, 16, 3)
        # an affine operation: y = Wx + b
        self.fc1 = nn.Linear(46656, 144)  # 6*6 from image dimension
        self.fc2 = nn.Linear(144, 84)
        self.fc3 = nn.Linear(84, 10)
  def forward(self, x):
        # Max pooling over a (2, 2) window
        x = F.max_pool2d(F.relu(self.conv1(x)), (2, 2))
        # If the size is a square you can only specify a single number
        x = F.max_pool2d(F.relu(self.conv2(x)), 2)
        x = x.view(x.shape[0], -1)
        print(x.shape) # prints flatten shape
        x = F.relu(self.fc1(x))
        x = F.relu(self.fc2(x))
        x = self.fc3(x)
        return x
def num_flat_features(self, x):
        size = x.size()[1:]  # all dimensions except the batch dimension
        num_features = 1
        for s in size:
            num_features *= s
        return num_features

It does not throws error when you have input t = torch.randn([1, 3, 224,224]). However if input size is different it might be problem, you should use transforms in Dataloader to reduce size to applicable for model.