太阳很大。

You can use this as a standalone observation. Just look out the window and sigh. You can also use it as a reason for an action. For example, use it to explain why you need an umbrella. Or why you are wearing sunglasses inside like a celebrity. It usually follows the pattern: [Subject] + [Adverb] + [Adjective]. Here, 太阳 is the subject and 很大 is the description. It is the bread and butter of small talk.

Use it whenever the light is blinding. It is perfect for weather updates with friends. Use it when planning a hike or a beach trip. It is a great way to start a conversation with a stranger at a bus stop. If you see someone sweating, this phrase is your best friend. It shows you empathize with their struggle against the heat. It is also a valid excuse to stay indoors and play video games.

Do not use this if it is just hot but cloudy. If the air feels like a sauna but you cannot see the sun, use 天气很热 (the weather is hot) instead. Avoid using it at night for obvious reasons. Unless you are making a very confusing joke about the moon. It is a bit too literal for poetic descriptions of a sunset. In those cases, you might want more flowery language. Keep it for the functional, bright daylight hours.

In many Chinese cultures, people go to great lengths to avoid the sun. Pale skin is often traditionally associated with beauty and a comfortable life. You will see people using umbrellas on perfectly clear days. This isn't for rain; it is because 太阳很大. Sunscreen, hats, and "face-kinis" are common sights at the beach. Saying the sun is "big" is often a warning to protect your skin. It is a shared social signal to seek shade immediately.

You can swap 很 (very) for other intensity markers. Use 太 (too) to say 太阳太大了 when you are complaining. Add 今天 (today) at the start to be specific. If you want to sound more casual, just say 阳光好刺眼 (the sunlight is so piercing). But for a beginner or intermediate speaker, the standard version is perfect. It is clear, correct, and everyone will understand your pain.