“机会很小”中英语翻译这个“小”应该用什么？

不能用small, 可以用 little.

例如：There is little chance/hope that...
或者 Chances are slim that...

不行。用little

little
[ˈlitl]
adj.
小的, 幼小的, 矮小的
What a pretty little house it is!
多么小巧玲珑的房子啊！
(时间、距离)短的
She stayed here only a little while.
她在这里只停留了很短的时间。
琐碎的, 微不足道的
A few little things apart, I am very pleased with the result.
撇开一些小事不谈, 我对这结果很满意。
pron.
少量, 些许
adv.
少到几乎没有, 毫不, 难得, 很少
Little does he know what trouble he's in.
他对自己身处何种麻烦茫无所知。

用little.

little

比如小男孩，a little boy

如果是物品、东西之类的体积小，用small

slim

the chances are slim
there is only a slim chance that ....

small chance 并没有错

“机会”属可数名词，应该用few.

I have two chances to visit the Great Wall.

祝你进步！