Update size estimates in the Makefile

MIDI_ENABLE = no

   text    data     bss     dec     hex filename
      0   17080       0   17080    42b8 satan_midi.hex

MIDI_ENABLE = yes
MIDI_BASIC undefined
MIDI_ADVANCED undefined

   text    data     bss     dec     hex filename
      0   19494       0   19494    4c26 satan_midi.hex

      +2414 bytes (vs. MIDI_ENABLE = no)

MIDI_ENABLE = yes
      0   20846       0   20846    516e satan_midi.hex

      +1352 bytes (vs. MIDI_ENABLE = yes, MIDI_BASIC off, MIDI_ADVANCED
off)

MIDI_ENABLE = yes
#define MIDI_BASIC
#define MIDI_ADVANCED

   text    data     bss     dec     hex filename
      0   21292       0   21292    532c satan_midi.hex

      +1798 bytes (vs. MIDI_ENABLE = yes, MIDI_BASIC off, MIDI_ADVANCED
off)

Conclusion:
    +2400 to 4200, depending on config

keyboards/satan/keymaps/midi/Makefile

@@ -9,7 +9,7 @@ CONSOLE_ENABLE = no         # Console for debug(+400)
 COMMAND_ENABLE = yes        # Commands for debug and configuration
 NKRO_ENABLE = yes           # Nkey Rollover - if this doesn't work, see here: https://github.com/tmk/tmk_keyboard/wiki/FAQ#nkro-doesnt-work
 BACKLIGHT_ENABLE = yes       # Enable keyboard backlight functionality
-MIDI_ENABLE = yes            # MIDI support (+3800)
+MIDI_ENABLE = yes            # MIDI support (+2400 to 4200, depending on config)
 AUDIO_ENABLE = no           # Audio output on port C6
 UNICODE_ENABLE = no         # Unicode
 BLUETOOTH_ENABLE = no       # Enable Bluetooth with the Adafruit EZ-Key HID

quantum/template/rules.mk

@@ -61,7 +61,7 @@ SLEEP_LED_ENABLE ?= no       # Breathing sleep LED during USB suspend
 # if this doesn't work, see here: https://github.com/tmk/tmk_keyboard/wiki/FAQ#nkro-doesnt-work
 NKRO_ENABLE ?= no            # USB Nkey Rollover
 BACKLIGHT_ENABLE ?= no       # Enable keyboard backlight functionality on B7 by default
-MIDI_ENABLE ?= no            # MIDI controls
+MIDI_ENABLE ?= no            # MIDI support (+2400 to 4200, depending on config)
 UNICODE_ENABLE ?= no         # Unicode
 BLUETOOTH_ENABLE ?= no       # Enable Bluetooth with the Adafruit EZ-Key HID
 AUDIO_ENABLE ?= no           # Audio output on port C6