Hi, welcome to the forum!
It´s difficult to say exactly which amount is apropriate for a particular newt. Even small species can tackle a rather big worm. It´s a bit of a trial an error thing, really.
You can read about the experiences of other people around the forums, and you can also find info here:
Caudata Culture Species Entry - Cynops orientalis - Chinese firebelly
I would recommend you to consider other species, though. I´m guessing you were thinking on purchasing H.orientalis because they are so commonly available. The truth is that all newts you see at pet-shops are wild-caught, and the importation process is a dreadful thing. As a result, by purchasing those animals you risk getting severely stressed individuals which can often become ill. It also perpetuates the mass importation of even more newts.
If you would consider a different species, one which is captive bred and easy to care for, i really think it would make the whole experience of owning newts a lot easier for you. Captive bred animals are already accostumed to captivity, have no impact on wild populations, adapt easily and accept a variety of foods. There are a number of excellent choices for beginners, like Pleurodeles waltl, Ambystoma mexicanum (axolotls), Tylototriton verrucosus, Triturus cristatus/carnifex/dobrogicus/karelinii or Hypselotriton cyanurus. All of those are very easy to keep, very tough and available from breeders.
Also, before you acquire any animals, make sure to research their requirements, read as much as you can and have everything ready (cycled tank, food cultures..). You´ll avoid lots of possible problems that way.