Use of the food diary?

Awsums

So, I've just had breakfast: A bowl of Swiss muesli with milk (whole for more bang per unit buck).
How do I work out what the weight of the serving was?

Also thinking I should quit buying milk, replace it with water, lemon juice, and chilli in cereal, tea, and coffee respectively.

Souba

It's hard to say with out measuring it. But on the side of the cereal it should mention amount per serving. (usually 30g). This is the same amount you would get in the little mini kid boxes of cereal if that gives you an idea of the size. Other than that, you should invest in a cheap set of kitchen scales to weight stuff out.

I don't think you should necessarily replace milk unless you're finding it really hard to hit those low daily calorie targets. Skimmed milk especially is very low in fat and yet provides a great source of protein along with vitamin D and calcium. Vitamin D you'll be hard pushed to find as it does not occur naturally in any foods, it is produced in our bodies when we're exposed to sunlight however, milk is fortified with it since calcium helps improve its absorption. All in all it's a very wholesome food.