Pubnub example for Atmel SAM D21 and WINC1500 is now part of ASF.
Look for updated code in ASF distribution, path
common/components/wifi/winc1500/pubnub_demo.
The Pubnub client is derived from the Contiki OS client, using the winc socket interface.
Its source code is in Pubnub.c. In the header (Pubnub.h) there are a few (documented) macros that the user can change to suit a particular application. For buffer/message size constraints, look at PUBNUB_BUF_MAXLEN and PUBNUB_REPLY_MAXLEN.
This Pubnub client support only the basic features: publish and subscribe to a channel (or a list of channels, but no channel group(s)) and leave (a (list of) channel(s)), which is suitable for most embedded applications.
Detailed instructions on how to connect all the HW, compile & upload the SW/FW and run the examples are in examples/README.md. Also, take a look at the application note in ASF.