So from what I understood, the Conv layer’s do not specify the image input size?

yes, they do not specify the input size. the input channel of the 1st conv layer has to match the number of channels for the image though. but they do affect the output tensor shape.

How do I calculate the matching dimensions of  nn.Conv  especially the first one, knowing I have a 1 channel 48 * 48 image? Or is that not relevant as a question?

I’m not sure what you mean. you mean how should you choose the parameters of the filters? There’s a bit of math required for this, you should read up on cs231n. If you are asking how to calculate the output tensor shape, then the formula for that is on the conv2d documentation