As you can see in Fig. 2 of the WaveNet paper the receptive field is 5, but the input size is larger (16). The receptive field defines what a single output neuron can see (see arrows in Fig. 2).
The receptive field could also be greater than the input, e.g. if you want to use or you only have the last 12 time steps and use the following structure (WaveNet paper, Fig 3), which can cover different powers of two depending on the number of layers.
If you want to calculate not only the last output neuron, it can make sense that the input size is larger than the receptive field, as the outputs before use also older inputs (see dashed lines).