Were talking apples and oranges. DXF is a VECTOR format where the
other formats you mentioned are RASTER formats. VECTOR does not have
any defined resolution or image size because it is a geometrical
description where RASTER is simple bit-mapped, like an array of
colored tiles (pixels) with a width and height defined in # of
pixels. You need somthing more powerful than an simple conversion
routine and you will need to interact with the conversion by
indicating the size of the raster image you want to create and the
color of the background and vectors as well as the depth of the image
you're creating (number of total colors definable: 2, 16, 256,
ect.). If you have access to AutoCAD, it provides all of this by
allowing raster formats to be defined as an output device.