Other than the order of blocks Squarespace doesn't offer a huge amount of control over the size of blocks on mobile.
This page describes your available options:https://support.squarespace.com/hc/en-us/articles/205815288
This topic is quite subtle in the way it works, so I'd recommend you talk with Squarespace support to see if they tweek that Facebook block. I'd be interested in knowing how you/they get the Facebook block looking right so please post an update.