23 kg is a weight limit, not a size: it's the usual maximum for checked luggage on most traditional airlines. There's no fixed size tied to that weight.
A quality medium suitcase (65-69 cm) usually weighs between 2.8 and 3.5 kg empty, leaving around 19-20 kg of real usable capacity for your clothes and belongings. A large suitcase (75-82 cm) has more room, but it's also easier to go over 23 kg if you pack it completely full.
The practical tip: weigh your suitcase at home before you leave. The excess weight fee at the airport is always more expensive than adjusting your luggage calmly beforehand.
Related questions: What's the ideal suitcase size for travelling? · What does it mean for a suitcase to be 40x30x20? · Complete guide to choosing your suitcase size
