It looks like the version of "size" used for AVR has a special output mode that isn't present for the ARM version:
avr-size[color=red] -C [/color]--mcu=atmega328 *.elf
AVR Memory Usage
----------------
Device: atmega328
Program: 10296 bytes (31.4% Full)
(.text + .data + .bootloader)
Data: 2052 bytes (100.2% Full)
(.data + .bss + .noinit)
BillW-MacOSX-2<10269> /Applications/arduino/Arduino-1.8.2.app/Contents/Java/portable/packages/arduino/tools/arm-none-eabi-gcc/4.8.3-2014q1/bin/arm-none-eabi-size -C *.elf
[color=red]arm-none-eabi-size: invalid option -- C[/color]
Usage: /Applications/arduino/Arduino-1.8.2.app/Contents/Java/portable/packages/arduino/tools/arm-none-eabi-gcc/4.8.3-2014q1/bin/arm-none-eabi-size [option(s)] [file(s)]
:
The options are:
-A|-B --format={sysv|berkeley} Select output style (default is berkeley)