PHP, 17 Bytes

<?=PHP_INT_SIZE*8

This correctly handles all of the cases of 32, 64 and other bit CPUs (Even though PHP only runs on 32 and 64 bit CPUs (But I have since been told that is an illegal loophole on codegolf (I didn't know that previously))).

provided that PHP_INT_SIZE is correct then it will show the precise size of the CPU no matter what CPU php is running on!

If PHP is running on

32-bit OS PHP_INT_SIZE == 4,

64-bit OS PHP_INT_SIZE == 8,

16-bit OS PHP_INT_SIZE == 2 (theoretically)

8-bit OS PHP_INT_SIZE == 1 (again theoretically)

128-bit OS PHP_INT_SIZE == 16 (Not yet achieved but possible)