Prawn - how to fit image to full size of PDF page

[stag...@nycopportunity.nyc.gov]

I'm currently working using prawn. 

      pdf_prawn = Prawn::Document.new(page_size: 'LETTER')

      pdf_prawn.bounding_box([0, pdf_prawn.cursor], width: 610.00, height: 790.00) do

        pdf_prawn.image format_image, fit: [:width, :height]

      end

I'm currently getting the error `NoMethodError (undefined method `to_f' for :height:Symbol)`

Before I try using 

pdf_prawn.image format_image, fit: [400, 400], position: :center, position: :center

Even though I was able to fill the whole image into the PDF I don't think this is the best solution.

I would like to know how to fit in the PDF the whole file without it being cropped.

from the two images above you can see that the size is not fitting 100 percent I wonder whats the best approach to fit the original file or get close to it.