In my opinion it depends on your goal. I like mine to be between 320 and 600. But if your goal is to simply break your fast… 40 calories will do that. The key is to make sure your acid levels stay down and maintain a balanced Micro and Macro nutrient diet.
I think you should eat what feels right for yourself. Although the best is probably a bit less than a third of your daily intake, if it feels like too much you can probably take it easy until it feels more natural.
i eat baguette with cheese & next fruits mixed with honey and then ride my bike to the coffeshop where i read a couple of pages from an inspirational book while iam drinking my nescafe .
It varies a lot, depending on your body type. Personally, I've struggled with weight gain – so I eat a bowl of granola for breakfast, then eat a large lunch and dinner with many snacks. If you are trying to lose weight, you could eat a low-fat parfait (yoghurt, granola, fruit). It tastes good, and has a lot of nutrients.
I'm sure there's a science to it but personally the only, even slightly successful, diet I've ever been able to stick to consistently is intermittent fasting. So I eat zero calories for breakfast every day. In fact I really only eat one meal a day after 6pm. I've been doing that for a couple months at a time and I'm still alive