The proper backpack size for a 25-kilometer hike per day will depend on a variety of factors, such as the duration of the hike, the weather conditions, the gear and equipment you need to carry, and your personal preference.
In general, a backpack size of 30-40 liters should be sufficient for a day hike of 25 kilometers. This size can comfortably hold essentials such as food, water, first aid kit, navigation tools, and extra clothing layers.
If you plan to carry more gear, such as a tent, sleeping bag, stove, and cooking supplies, you may need a larger backpack. A backpack with a capacity of 50-70 liters would be more appropriate for a multi-day hike or a hike where you need to carry more gear.
It is important to note that regardless of the backpack size, it is essential to pack only what you need and keep the weight of your pack as light as possible to avoid unnecessary strain on your back and shoulders.