I’m hoping there will be an easier way to change this from the build IDE, but if you’re compiling locally ( https://github.com/spark/core-firmware ) then you could change the UDP buffer to be any size you like! 
It sounds like you’re already familiar with this, but in case you haven’t setup the local build, it’s fun to be able to change anything you want. Mohit made a great tutorial here: https://community.spark.io/t/the-spark-core-firmware/532
//here -> https://github.com/spark/core-firmware/blob/master/inc/spark_wiring_udp.h
#define RX_BUF_MAX_SIZE 512
Thanks!
David