The bitmap size you get depends on the screen area and resolution since it is effectively a "dump" of the graphics in memory. The image will normally be OK at the original size, but bitmaps do not scale up well beyond a low number of %. As they scale down important details can easily get lost. Therefore it is important to ensure the output size you want is as close as possible to the size you will use. If necessary this means multiple exports and then stitching à la una panorama. :)
DXF is the export vector format supported. Vectors scale better. Typically that output can be used by CAD programs like TurboCAD or Sketchup (free). Every selected visible line when you export in the layout is a DXF object - so things like whether sleepers are shown make a difference to the complexity of the output. Before V5.2, every sleeper is 4 DXF objects. Note this is a 2D output - elevation is not included.
Another approach is print to PDF. PDF is at a fixed scale like a bitmap but has the lower file size of DXF because the lines and text are being exported as objects and not bits - some programs can cope with that as input and edit it, notably Illustrator can use a PDF variant as their storage type, so they can import it, edit it and then export as a bitmap rescaled as you wish. If you do this, you would set the PDF "printer paper size" as large as you can and also you can set the resolution up to 600 lpi or more.
Oh, and yes, you need to select everything you want included first for DXF.
Hope that helps,