This post is now a couple years old, so we've grown a lot since then. Here's a newer (more marketing-centric) post where we announced our 850k/second milestone: https://onesignal.com/blog/throughput-record/

It's generally under 4 bytes per delivery, depending on the content, and we have several delivery servers. APNS, for example, doesn't support payloads larger than 4 bytes.