@Shantharao 

you can use after insert BR on incident table and then query sys_attachment and know the size and accordingly update the work notes

Business Rule: After Insert on incident

Script:

(function executeRule(current, previous /*null when async*/) {
    var grAttachment = new GlideRecord('sys_attachment');
    grAttachment.addQuery('table_name', 'incident');
    grAttachment.addQuery('table_sys_id', current.sys_id);
    grAttachment.addQuery('size_bytes', '>', 3145728); // 3 MB in bytes
    grAttachment.query();
    var largeAttFiles = [];
    while (grAttachment.next()) {
        largeAttFiles.push(grAttachment.file_name + ' ('+(grAttachment.size_bytes/1048576).toFixed(2)+' MB)');
    }
    if (largeAttFiles.length > 0) {
        current.work_notes = 'The following attachments uploaded are larger than 3MB:\n' + largeAttFiles.join('\n');
		current.setWorkflow(false);
        current.update();
    }
})(current, previous);

If my response helped please mark it correct and close the thread so that it benefits future readers.