I learned on straight U.S. size 10 plastic needles. They say 6 mm but as they are Boye's I'm not sure I trust that, but it's probably accurate. They aren't great needles, they are inexpensive and the plastic, like bamboo, will help keep the stitches on the needle when you want them there, bamboo will do it better.
If you are looking at needles online, maybe you could link to them and then we could see what the price is, could you do better somewhere else, etc.
It's great that you're asking before you buy small needles and heavy yarn and can't get it to come out right.