Parece que o manual do tamanho GNU não especifica, mas acredito que eles estejam em bytes para todas as versões de tamanho.
Aqui está o manual da Apple size :
SIZE(1) SIZE(1)
NAME
size - print the size of the sections in an object file
SYNOPSIS
size [ option ... ] [ object ... ]
DESCRIPTION
Size (without the -m option) prints the (decimal) number of bytes required by the __TEXT, __DATA and __OBJC segments. All other seg-
ments are totaled and that size is listed in the 'others' column. The final two columns is the sum in decimal and hexadecimal. If no
file is specified, a.out is used.
The options to size(1) are:
- Treat the remaining arguments as name of object files not options to size(1).
-m Print the sizes of the Mach-O segments and sections as well as the total sizes of the sections in each segment and the total
size of the segments in the file.
-l When used with the -m option, also print the addresses and offsets of the sections and segments.
-x When used with the -m option, print the values in hexadecimal (with leading 0x's) rather than decimal.
-arch arch_type
Specifies the architecture, arch_type, of the file for size(1) to operate on when the file is a universal file. (See arch(3)
for the currently know arch_types.) The arch_type can be "all" to operate on all architectures in the file. The default is to
display only the host architecture, if the file contains it; otherwise, all architectures in the file are shown.
SEE ALSO
otool(1)
BUGS
The size of common symbols can't be reflected in any of the numbers for relocatable object files.
Apple Computer, Inc. July 28, 2005 SIZE(1)
E o manual SmartOS size :
SIZE(1) User Commands SIZE(1)
NAME
size - print section sizes in bytes of object files
SYNOPSIS
size [-f] [-F] [-n] [-o] [-V] [-x] filename...
DESCRIPTION
The size command produces segment or section size information in bytes
for each loaded section in ELF object files. size prints out the size
of the text, data, and bss (uninitialized data) segments (or sections)
and their total.
size processes ELF object files entered on the command line. If an
archive file is input to the size command, the information for each
object file in the archive is displayed.
When calculating segment information, the size command prints out the
total file size of the non-writable segments, the total file size of
the writable segments, and the total memory size of the writable
segments minus the total file size of the writable segments.
If it cannot calculate segment information, size calculates section
information. When calculating section information, it prints out the
total size of sections that are allocatable, non-writable, and not
NOBITS, the total size of the sections that are allocatable, writable,
and not NOBITS, and the total size of the writable sections of type
NOBITS. NOBITS sections do not actually take up space in the filename.
If size cannot calculate either segment or section information, it
prints an error message and stops processing the file.
OPTIONS
The following options are supported:
-f
Prints out the size of each allocatable section, the name of the
section, and the total of the section sizes. If there is no
section data, size prints out an error message and stops
processing the file.
-F
Prints out the size of each loadable segment, the permission
flags of the segment, then the total of the loadable segment
sizes. If there is no segment data, size prints an error message
and stops processing the file.
-n
Prints out non-loadable segment or non-allocatable section sizes.
If segment data exists, size prints out the memory size of each
loadable segment or file size of each non-loadable segment, the
permission flags, and the total size of the segments. If there is
no segment data, size prints out, for each allocatable and non-
allocatable section, the memory size, the section name, and the
total size of the sections. If there is no segment or section
data, size prints an error message and stops processing.
-o
Prints numbers in octal, not decimal.
-V
Prints the version information for the size command on the
standard error output.
-x
Prints numbers in hexadecimal, not decimal.
EXAMPLES
The examples below are typical size output.
Example 1 Producing size information
example% size filename
2724 + 88 + 0 = 2812
Example 2 Producing allocatable section size information
example% size -f filename
26(.text) + 5(.init) + 5(.fini) = 36
Example 3 Producing loadable segment size information
example% size -F filename
2724(r-x) + 88(rwx) + 0(rwx) = 2812 ... (If statically linked)
SEE ALSO
as(1), ld(1), ar.h(3HEAD), a.out(4), attributes(5)
NOTES
Since the size of bss sections is not known until link-edit time, the
size command will not give the true total size of pre-linked objects.
August 24, 2009 SIZE(1)
Mais informações sobre a saída de size podem ser encontradas aqui