ZF-4394: Zend_Validate_File_Size: Error message template variables
In Zend_Validate_File_Size I reckon it'd be great if _throw was setting _min and _max to the respective values of the getMin() and getMax() methods. Since both methods call _toByteString it would result in friendlier error messages when they make use of the %min and %max placeholders.
I would also suggest breaking the loop in _toByteString when $size >= 1024 (instead of $size > 1024), 1MB is imo more user friendly than 1024kB.
Also in isValid() there is $this->_setValue($size) which sets %value to a file size. It is a bit confusing since the default error messages more or less treat %value as a file name. Even though it's overwritten in _throw when $files is set it might surprise some people that use this validator manually. (Zend_Validate_File_FileSize and Zend_Validate_File_ImageSize also do something similar).
Last suggestion, maybe a %size placeholder set to the actual size of the tested file could be useful to some people.