Let’s calculate the output size manually.
Your input length is 800 and your kernel_size is 400.
Using a stride of 200 this would result in 3 output values.
The output values would be calculated from the following input windows: input[0:400], input[200:600], input[400:800].
Since you are using a dilation of 2, your kernel_size is spread to a size of 800.
Therefore, you only get one output, since the input size is now equal to your “active” kernel_size, i.e. both are 800.
Have a look at @ezyang convolution visualizer 147 and use your setiings to get a feeling about the operation.
@ezyang Setting input size or stride to 0 lets the page disappear.  Awesome tool btw!