I have found that a 42mm objective lens is a great size for gathering light while remaining fairly small and lightweight. So a 8X42 or 10X42 is my preference.
When you go with a smaller lens than 42mm you are losing good light gathering ability and as a result, they won't perform very well in low light conditions, which is exactly when you need them to perform.
When you go with a larger objective lens, you get better light gathering capability, but you start to put on weight, which can be tiring if you are using the binos all day.