Hello,
I feel this is one of the important setting, which was restricted due to packet size. So I have made this plugin to allow UNLIMITED Storage Size.
Just enable this plugin and set MAX_STORAGE(from common/mmo.h) to whatever you want.
Link: https://github.com/dastgir/HPM-Plugins/blob/master/src/plugins/storage_expand.c
Note: This doesn't work with Guild Storage.