If you're wondering how much luggage to take on your journey you're asking the wrong question. The fact is you will fill up whatever bag you use for the journey, so the correct question is "What bag should I use?"
If you're thinking "I should make a list of all the things I'll need and then choose a bag" you're taking the wrong approach and will end up in Stressville. You don't actually need any luggage at all. The only things you really need are passport and wallet; everything else you can buy when you get there.
But it's really hard to get to that state of mind, that place of Zen, where you can travel with nothing but your passport and credit card -- especially if you're bogged down by the endless details of job and family and have a house full of things that should be used once in a while.
Whenever you find yourself saying "this might be useful" you should realise that it will almost certainly not come in useful and you should leave it behind (in your house full of other things that might be useful).
When I first went to India I took a very small rucksack, a daypack, which contained a T-Shirt, a pair of shorts, a cloth that was my towel and bed-sheet, a diary and a book. It was the optimum amount of gear and I had no problems. Some years later I went back with a big rucksack, containing many more clothes and I can't remember what else, but there was no qualitative improvement.
Now that I am a father of two wonderful kids and have a full time job I find it really hard to not take too much luggage. I go to the UK frequently and even though I know that I don't actually need anything, as I will probably be staying with my parents where I have plenty of old clothes stashed away, I end up filling a massive rucksack with clothes, presents, books and a sleeping bag and mat (in case I end up roughing it, which is more of a wish than a reality).
If I go camping with a rucksack I will fill it up but if I go camping with a car I take about three times more stuff, because a car is like a vast suitcase on wheels.
Last weekend I went on a short cycle tour along the coast of Romania and Bulgaria and took far too much stuff -- tent, cooking gear, food, waterproofs -- and my bike was almost 15kg heavier, making the journey a lot tougher than it should have been. This weekend I am going to travel light, leave behind the camping gear and just take the minimum (waterproofs).
Already I can feel my mind trying to cheat the system. Yesterday I decided to ensure this minimal approach by just taking two out of the four bags I can attach to my bike, and just take essentials like bike repair gear. But the workaholic part of my brain is saying "if you're taking so little it won't make any difference to take the laptop -- think of all that stuff you can write?" And then I will need an extra bag and I'll end up with too much stuff again.
What about all those things you normally need on holiday? Like cameras, chargers, books, laptops, notebooks, umbrellas, shoes, cosmetics, hats, different clothes, playthings...the list is endless. All you can do is tell yourself you don't really need all that stuff -- and choose a smaller bag.
I love to travel with books but I now have a Kindle; I also love to travel with a camera but mine is too big so it will stay at home. If you have an iPhone you have all the technology you need already in your pocket.
Travelling light isn't easy as we have an enemy within that's constantly telling us to take more. But if you follow this simple rule you can prevail.