BenchmarkSwitcher.FromAssembly(typeof(Tests).Assembly).Run(args);

public class Tests
{
    public IEnumerable<object> GetMultiplyArgs()
    {
        Random random = new Random(227);
        var lengths = new int[] { 1000, 10000, 100000, 1000000 };
        for (int i = lengths.Length - 1; i >= 0; i--)
        {
            var largeBytes = MakeBytes(random, lengths[i]);
            var large = new BigInteger(largeBytes, isUnsigned: true);
            {
                var smallBytes = MakeBytes(random, lengths[i] / 2);
                var small = new BigInteger(smallBytes, isUnsigned: true);
                yield return new Data($"{largeBytes.Length:D7}-{smallBytes.Length:D7}", large, small);
            }
            for (int j = i; j >= 0; j--)
            {
                var smallBytes = MakeBytes(random, lengths[j]);
                var small = new BigInteger(smallBytes, isUnsigned: true);
                yield return new Data($"{largeBytes.Length:D7}-{smallBytes.Length:D7}", large, small);
            }
        }
        static byte[] MakeBytes(Random random, int length)
        {
            var bytes = new byte[length];
            random.NextBytes(bytes);
            return bytes;
        }
    }

    public record Data(string Name, BigInteger Large, BigInteger Small)
    {
        public override string ToString() => Name;
    }

    [Benchmark]
    [ArgumentsSource(nameof(GetMultiplyArgs))]
    public BigInteger Multiply(Data data)
    {
        return data.Large * data.Small;
    }
}