You could compute the data size simply with 5000*521*265*nb_channels*bytes_per_element.
Besides that your model will of course use memory on the device (parameters + intermediate tensors for backpropagation).
That being said, usually you would use mini-batches instead of loading the complete dataset to the GPU, so save the memory for a larger model.
What’s your use case that you want to load the complete data?