Solved by Toby in post #2 
              
              
                For the first question: 
Run print(model), you should see the first layer is (conv1): Conv2d(3, 64, kernel_size=(7, 7), stride=(2, 2), padding=(3, 3), bias=False). You modify your first layer without padding and stride, that why it became 26. 
For the second question: 
Because this is CNN models, so…