It is possible to define this constant in your file 
config/mes_options.php (see the article dedicated to it)
 Default value: 0
example :
// logos larger than 350 Kb will not be registered.
define('_LOGO_MAX_SIZE', 350);
When this constant is set to 0 (zero), SPIP does not carry out a check when the image is uploaded.
See also: _LOGO_MAX_WIDTH and _LOGO_MAX_HEIGHT.