SerializerInfo.cs

public class SerializerInfo
{
    const double EPSILON = 0.0001;
    readonly object _lock = new object();
    double _averageSize = 0;
    long _count = 0;

    public SerializerInfo(XmlSerializer serializer)
    {
        Serializer = serializer;
    }

    public SerializerInfo AddIntanceSize(int size)
    {
        lock (_lock)
        {
            var delta = (size - _averageSize) / _count;
            if (Math.Abs(delta) < EPSILON)
                return this;
            _count++;
            _averageSize += delta;
            return this;
        }
    }

    public long AverageSize => (int)Math.Ceiling(_averageSize);

    public XmlSerializer Serializer { get; }
}