Bitfury's paper here:

http://bitfury.com/content/4-white-papers-research/block-size-1.1.1.pdf

"The table contains an estimate of how many full nodes would no longer function without hard-ware upgrades as average block size is increased. These estimates are based on the assumption that many users run full nodes on consumer-grade hardware, whether on personal computers or in the
cloud. Characteristics of node hardware are based on a survey performed by Steam [19]; we assume PC gamers and Bitcoin enthusiasts have a similar amount of resources dedicated to their hardware.

The exception is RAM: we assume that a typical computer supporting a node has no less than 3 GB RAM as a node requires at least 2GB RAM to run with margin[15]. For example,if block size increases to 2 MB, a node would need to dedicate 8 GB RAM to the Bitcoin client, while more than a half of PCs
in the survey have less RAM."

Based on his estimation, raise the block size to 4MB will drop 75% of nodes from the network