How to initialize a List<T> to a given size (as opposed to capacity)?

List<string> L = new List<string> ( new string[10] );

I can't say I need this very often - could you give more details as to why you want this? I'd probably put it as a static method in a helper class:

public static class Lists
{
    public static List<T> RepeatedDefault<T>(int count)
    {
        return Repeated(default(T), count);
    }

    public static List<T> Repeated<T>(T value, int count)
    {
        List<T> ret = new List<T>(count);
        ret.AddRange(Enumerable.Repeat(value, count));
        return ret;
    }
}

You could use Enumerable.Repeat(default(T), count).ToList() but that would be inefficient due to buffer resizing.

Note that if T is a reference type, it will store count copies of the reference passed for the value parameter - so they will all refer to the same object. That may or may not be what you want, depending on your use case.

EDIT: As noted in comments, you could make Repeated use a loop to populate the list if you wanted to. That would be slightly faster too. Personally I find the code using Repeat more descriptive, and suspect that in the real world the performance difference would be irrelevant, but your mileage may vary.

Use the constructor which takes an int ("capacity") as an argument:

List<string> = new List<string>(10);

EDIT: I should add that I agree with Frederik. You are using the List in a way that goes against the entire reasoning behind using it in the first place.

EDIT2:

EDIT 2: What I'm currently writing is a base class offering default functionality as part of a bigger framework. In the default functionality I offer, the size of the List is known in advanced and therefore I could have used an array. However, I want to offer any base class the chance to dynamically extend it and therefore I opt for a list.

Why would anyone need to know the size of a List with all null values? If there are no real values in the list, I would expect the length to be 0. Anyhow, the fact that this is cludgy demonstrates that it is going against the intended use of the class.