The operating system does not know where to find executables unless either
1. The file is in a directory defined by the %path% environment variable
or
2. You give it the complete path name as part of the command.
And, yes, you are correct: avr-size.exe (and other avr-stuff) is under the arduino installation directory:
\whatever\arduino-0022\hardware\tools\avr\bin
Append that directory to your %path%. (In Windows XP, you can set and modify environment variables from the Control Panel->System->Advanced->Environment Variables box. I don't know exact details for later versions of Windows.)
avr-size should be included in the avr-gcc toolchain that i suppose comes with Arduino IDE! Am I right?
yes, but the ide invokes the programs explicitly, and does NOT modify your default system paths to include all of the binary directories that it provides.
Like Dave said: \whatever\arduino-0022\hardware\tools\avr\bin
For example, I have a \bin\ directory where I put less formal installs, and I can do:
C:\Documents and Settings\Bill Westfield>c:\bin\arduino21\hardware\tools\avr\bin\avr-size.exe --help
Usage: c:\bin\arduino21\hardware\tools\avr\bin\avr-size.exe [option(s)] [file(s)]
Displays the sizes of sections inside binary files
If no input file(s) are specified, a.out is assumed
The options are:
-A|-B|-C --format={sysv|berkeley|avr} Select output style (default is berkeley)
--mcu=<avrmcu> MCU name for AVR format only
-o|-d|-x --radix={8|10|16} Display numbers in octal, decimal or hex
-t --totals Display the total sizes (Berkeley only)
--common Display total size for *COM* syms
--target=<bfdname> Set the binary file format
@<file> Read options from <file>
-h --help Display this information
-v --version Display the program's version
Within a particular session, the cmd window history functions are very useful. If you're going to be doing this a lot, you can add the appropriate directory to your path:???