it is best to use an H or I size hook
You can use a bigger hook, or use a thicker thread or yarn. If you know, or can figure out, the multiple of the pattern, you can increase the foundation chain and make it larger in that manner too. If the pattern is completed using squares, you can increase the size of each square or use a joining method that adds width and length to the project. You could also increase the size by adding a multiple round edging around your finished piece.
Your answer depends on the size of the crochet needle you want to use. First, crochet a gauge, so that you have the basic information you need to calculate your raw material requirements.
look at the pattern...... it usually says how much..... i.e.: 5 skeins of yarn As noted above, the pattern generally states how much yarn to use for that pattern. If you are trying to design your own pattern, however, then the process is more complicated. There is no general rule for how much yarn in a pattern, even as to type of pattern, because each is so different. Different scarf patterns, for example, would take different amounts of yarn. Generally speaking, I can probably make a scarf if I have 7 to 10 ounces of yarn. If I am making a hat for a child, I can do it with one 3.5 ounce skein with yarn leftover. For an adult, it depends on the pattern. Sometimes one skein would be enough, sometimes two would be needed. Afghans require more yarn, but amount varies with yarn, hook, and the size of the afghan as well as the pattern. One can take a baby afghan pattern and use chunky yarn to give a larger afghan, but amount of yarn required will be different. If you have a pattern and it gives you the yarn amount in skeins but not ounces, and you want to use a different yarn, then you need to find out how many ounces in that type of yarn, then buy the required ounces in a yarn in the same weight category.
Your answer depends on the gauge that you knit with the yarn and needles that you want to use and the size of the afghan that you want to make.Can you do the math?
The size of the needle depends on the gauge of the pattern you are using. If you are not using a pattern, or are using a pattern that has no gauge, then check the label of the yarn used. Most labels today will give you a size of needle that they recommend for that specific yarn.
that would be a size P crochet hook
For yarn, use an "E" or an "F".
In UK sizes, a 2mm crochet hook is a 14, it doesn't not have a US size equivalent. Here is a chart with crochet hook conversions from metric to US to UK.
A 5.0mm would be an H-8 hook.
It depends on the size of yarn used. For worsted weight size 4 yarn, it reccommends to use a size g or h hook. Check the yarn cover to see which size hook is reccommended for the yarn.
This varies with the yarn, the hook size and the tension. You have to make a sample, count and measure.
A 0.9 mm steel hook is about a US size 13.
A size 5 steel hook. http://www.crochetmagazine.com/hook_sizes.php
A 3mm crochet hook is halfway between a C and D size in US sizes.
It's not totally clear here what you are asking. If you are asking if you can get a 50mm (approximately 2 inches) crochet hook, I think you could--but what are you making that would need a broomstick-sized hook? Are you looking to find a "size 59 metric crochet hook"? It wouldn't be easy, the largest hook I've seen has been a size 'Q' which is 15mm. If you want one bigger, you would have to build it yourself, would be my guess.
00 is a steel hook size. Manufacturers vary in their mm sizing. I found 00 hooks listed as 2.75 to 3.5mm If you have a particular hook and you need to know the mm sizing of it, I suggest you purchase a Crochet Hook Size Gauge Tool. The knitting needle ones do not give accurate sizing for a crochet hook. Annie's Attic sells these and I'm sure other stores do as well.
A size "C" crochet is generally around 2.75 mm in the United States. There isn't really any specific standards regarding crochet hook sizes, different manufacturers made their hooks in different sizes, and that pretty much still is happening today.