save() on espruino_1v91.122_esp8266 results in immediate Disconnected prompt

you should place the code in some flash areas and use eval or pipe.

With that new board build you will have this areas:

var freeFlash = require('Flash').getFree()
=[
      { "addr": 2097152, "length": 1048576 },    // <- 1MB
      { "addr": 3145728, "length": 262144 },     // and some smaller sections
      { "addr": 3407872, "length": 262144 },
      { "addr": 3670016, "length": 262144 },
      { "addr": 3932160, "length": 245760 }
 ]