In general, the disclosure describes techniques for evaluating application quality of experience metrics over a software-defined wide area network. For instance, a network device may receive an application data packet of a data flow. In response to receiving the application data packet, the network device determines whether a packet size of the application data packet is represented in a reference data store. In response to determining that the packet size is not represented in the reference data store, the network device predicts, based on the reference data store, flow metrics for the packet size for each of a plurality of Wide Area Network (WAN) links. The network device selects a WAN link on which to send the application data packet based on the predicted flow metrics.