You should always use pdfwidth= for images in PDFs (which you can use along with the width attribute for HTML). The units can be relative (%) or absolute (in, cm, pt, etc)

 

9/10 this is an issue with prawn-svg. See https://github.com/mogest/prawn-svg

However, before you report it there, you may want to try using the latest version of prawn-svg, which is not yet installed by default (it will be once we cut a new alpha of asciidoctor-pdf).