Crochet hooks come in various sizes. To figure out a project that you might want to crochet, find the project, see what size yarn is needed, and then check with the pattern, and see what the pattern requires for a hook size.
As a general rule "steel" hooks are used for thread-weight "yarn" and those hooks run from 0.6 mm to around 3.0mm. You know that you have a "steel" hook, because it has a number on it, in addition to a size given in mm. 'Steel' hooks run from size 14 (0.6mm) to 00 (double zero, and 3.0mm)
Then the "aluminum" hooks run from around 2.0mm all the way up to 16.0mm or larger. An aluminum hook can be easily identified because there is a letter on the hook, along with the mm sizing. So, 'aluminum' hooks run from B (2.25mm) to S (19.0mm). I guess there could be an "A" size, but it would be around the same as a size Zero or Double Zero in 'steel' hooks.
Milliliters are not used for crochet hooks. They measure volume. You can use millimetres to measure the thickness of a hook.
A one millimetre steel hook is a US size 10.
that would be a size P crochet hook
A 5.0mm would be an H-8 hook.
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.
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.
In the US, knitting needle sizes are in numbers; crochet hooks use both numbers and letters.
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.
A 3mm crochet hook is halfway between a C and D size in US sizes.
A U.S. size 7 yarn hook is 4.5 mm. http://www.crochetmagazine.com/hook_sizes.php
A 0.9 mm steel hook is about a US size 13.
A size 5 steel hook. http://www.crochetmagazine.com/hook_sizes.php
For yarn, use an "E" or an "F".