Sure. In this case, you’d just do:
length_key = len(d['key']) # length of the list stored at `'key'` ...
It’s hard to say why you actually want this, but, perhaps it would be useful to create another dict that maps the keys to the length of values:
length_dict = {key: len(value) for key, value in d.items()}
length_key = length_dict['key'] # length of the list stored at `'key'` ...