Son Bytes ..
Pero lo mejor de todo esto no es que yo diga o que diga el otro .. Se trata de acudir a la documentación oficial de PHP y ver que dice allá .. es la mejor forma, la más rápida y la más segura. Crear habito de ir a mirar la referencia oficial antes de preguntar es bueno. (no lo digo como una "retada" o "regaño" .. sino como comentario general .. en serio que ayuda).
http://www.php.net/manual/en/features.file-upload.php Cita: The MAX_FILE_SIZE hidden field (measured in bytes) must precede the file input field, and its value is the maximum filesize accepted. This is an advisory to the browser, PHP also checks it. Fooling this setting on the browser side is quite easy, so never rely on files with a greater size being blocked by this feature. The PHP settings for maximum-size, however, cannot be fooled. This form element should always be used as it saves users the trouble of waiting for a big file being transferred only to find that it was too big and the transfer failed.
Un saludo,