size: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/size - from package: binutils

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/size - from package: binutils-multiarch
/usr/bin/size - from package: binutils

You may install the required package with command apt-get install {package}

Error in other languages:
size: bevel nie gevind nie
size: comandă negăsită
size: comando não encontrado
size: commande introuvable
size: command not found
size: käsku ei ole
size: không tìm thấy lệnh
size: komanda nerasta
size: Kommando nicht gefunden.
size: kommandot finns inte
size: komut yok
size: nie znaleziono polecenia
size: níor aimsíodh an t-ordú
size: no se encontró la orden
size: no s'ha trobat l'ordre
size: opdracht niet gevonden
size: parancs nem található
size: perintah tidak ditemukan
size: príkaz nenájdený
size: příkaz nenalezen
size: команда не найдена
size: командата не е открита
size: 命令找不到
size: コマンドが見つかりません