http://codereview.stackexchange.com/questions/126277/internal-function-that-determines-when-to-resize-by-width-or-height-in-a-image-r

ComputeSize

private Dimention2D ComputeSize(Image image, int? maxWidth, int? maxHeight)
{
    if (image == null) throw new ArgumentNullException(nameof(image));
    if (maxWidth <= 0) throw new ArgumentOutOfRangeException(nameof(maxWidth));
    if (maxHeight <= 0) throw new ArgumentOutOfRangeException(nameof(maxHeight));
    if (image.Width <= 0) throw new ArgumentOutOfRangeException(nameof(image.Width));
    if (image.Height <= 0) throw new ArgumentOutOfRangeException(nameof(image.Height));

    var sourceDimension = new Dimention2D(image.Width, image.Height);

    bool computeWidth = sourceDimension.Width > maxWidth;
    bool computeHeight = sourceDimension.Height > maxHeight;

    if (computeWidth && computeHeight)
    {
        if (sourceDimension.Width / maxWidth > sourceDimension.Height / maxHeight)
            computeHeight = false;
        else
            computeWidth = false;
    }

    if (computeWidth)
        return sourceDimension.ResizeWidth(maxWidth.Value);

    if (computeHeight)
        return sourceDimension.ResizeHeight(maxHeight.Value);

    return sourceDimension;
}