The following free C# sample code shows how to convert PDF to BMP image in Visual Studio .NET project. You may also get more details for .NET PDF to BMP converting properties and APIs here.
In addition, here's a C# coding sample for how to load a PDF from stream and transform it to the right size BMP image file. That is to say, you are empowered to customize the size of output BMP image according to your specific PDF document converting requirement.
If you want to resize BMP image and change its quality through DPI, you may refer to this online C# tutorial: How to customize (BMP) image converted from PDF page using C#.