1. "Why are beads etc, always described in millimetres?"
Well, quite simply because most of the world, notably the places where beads and findings are made, and most people who make jewellery, use the metric system. Even those who don't use metric in other aspects of their life are accustomed to buying beads in metric sizes, and it just becomes second nature, much as an electrician uses wire gauges when he's talking about his work.
Once you get used to the metric system, it's much easier, which is why it's so popular. The problem is, when you are not used to it, in visualizing the size of something when it is described. Considering just how important the metric system is in the modern world, I recommend obtaining a ruler marked in centimetres and millimetres for ALL shopping online. This is a much easier guide to have around than any amount of conversion charts. This is the future!
2. "How do I measure for a bracelet?"
Not as simple as it sounds, I've seen a few errors happen. Your wrist measurement is not the size you need. And it's no good adding on "a bit" for luck, as it were. What you should do is measure another bracelet that fits nicely. First, measure it open, laid out straight, and include the clasp. Then measure the inside of it when done up. Give these two measurements to your bespoke bracelet maker. Half an inch makes a HUGE difference with a bracelet, so it's important to measure accurately. If you don't have a tape measure to bend around the inside, use a piece of string, and then measure that on a straight ruler.
3. "It looked a different size in the photo."
When jewellery is photographed it is done so to capture its beauty. This may involve a whole necklace being seen in a small image, or a ring being blown up to several times actual size. Most people understand that much. Nevertheless I've had customers complain that an earring was much smaller than expected (I always list the size, but that's a separate matter). Now, this is despite the earwire being in the photo. Look at the screen. Hold your thumb up to it. The length of an earwire, in reality, is roughly the length of your thumbnail (or maybe a tad smaller if you have tiny hands). This will give you an idea of scale, regardless of the size of the image itself, or how it is displayed. If the earwire is TWICE the length of your thumbnail on-screen, then the rest of the earring is half the size of that displayed, in reality, also.
4. "Why is a bracelet only slightly less expensive than a necklace? Surely it should be half the price? It's only half the materials".
This might be fair if you were talking about a commercially-made gold chain. But with hand-made jewellery the materials are not the bulk of the price, the maker's time is. Depending on the design, the time it takes to make a bracelet may be as much as 2/3 the time to make a necklace. Then, when you do add on the materials, this can take it up to around 3/4.
If you have any questions, on this or any other topic, feel free to ask!