Is it meant by "his sins have been forgiven" only minor sins, or all sins, including major ones, or only minor sins of that particular day?

The sins referred to in the Hadith are minor sins, and this includes all minor sins committed by the servant, whether on the same day or before. As for major sins, one must repent from them before death. The word "sins" (dhunub) in Islamic law applies to both major and minor sins.