I tend to avoid doing a Select followed straight by a Max when you can just pass the select function directly into the max. Also multiline lambda expressions feel ugly but that's probably just me! This is what I would do these days:

    string pad(this string e, int padLength) => new String(' ', padLength) + e;
    var input = new string[] { "abc", "ab", "abcdef", "abcdefgh" };
    var maxLength = input.Max(e => e.Length);
    var output = input.Select(e => e.pad((maxLength - e.Length) / 2));