how to convert bitmap image size to double size

You cannot assign a double value there. You are using this signature of the Bitmap constructor.

Bitmap(Image, Int32, Int32);

http://msdn.microsoft.com/en-us/library/334ey5b7(v=vs.110).aspx[^], and it will never accept a double value in it.

You can however still convert the double to integer while passing it but passing a double value would not work. Or you can convert the int values returned by Width and Height properties to double for working in your code.

In WinForms Controls' 'Size and 'Location properties are integer only.

In WPF these properties use double.

The 'SizeF structure in WinForms System.Drawing is used almost always for special operations in the Paint Event.

However, if you are doing some re-sizing of Controls in WinForms and want increased accuracy based on ratios: calculate Float Type values before scaling, then convert the 'SizeF to 'Size (integer) to scale with:

// not a complete code example
// example of what you would do in an EventHandler for a Form 'SizeChanged event

float xRatio, yRatio;

xRatio = (float)(newFormSize.Width) / oldFormSize.Width;
yRatio = (float)(newFormSize.Height) / oldFormSize.Height;

SizeF newSize = new SizeF(xRatio * pictureBox1.Width, pictureBox1.Height * yRatio);

pictureBox1.Size = newSize.ToSize();

Complete code example on request: yes, using ratios like this does give more accurate scaling.