It would depend on the gauge you are trying to obtain. #8 crochet thread generally works up best with small steel hooks in sizes like 1.0-2.25mm but you could produce a lace with a nice drape using a larger hook, such as a 3.5mm or even 4.0mm or 5.0mm. If you have the time, make small swatches using various sized hooks and put them in a scrapbook for future reference.
that would be a size P crochet hook
A 3mm crochet hook is halfway between a C and D size in US sizes.
it is best to use an H or I size hook
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.
A 0.9 mm steel hook is about a US size 13.
A size 5 steel hook. http://www.crochetmagazine.com/hook_sizes.php
you don't cast on to crochet. you do a base chain. to answer this question, you need to take into account the size thread you are using and the size hook. why don't you do crochet a diagonal stitch afghan. when it gets to be 60" wide, you decrease. there is a book delma's diagonal stitch which you can get, or search for a free diagonal stitch pattern. good luck
For yarn, use an "E" or an "F".
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.