How can the size of the notification queue be configured?
I found this in async.c
 298 #define QUEUE_PAGESIZE              BLCKSZ
     ...
 300 
 301 /*
     ...
 310  * The most data we can have in the queue at a time is QUEUE_MAX_PAGE/2
 311  * pages, because more than that would confuse slru.c into thinking there
 312  * was a wraparound condition.  With the default BLCKSZ this means there
 313  * can be up to 8GB of queued-and-not-read data.

This supports the notion that the queue size is not configurable without changes to the source and recompilation, as a_horse_with_no_name commented.