There are two possible solutions for this issue: 

  1. The LTI configuration should be removed, then re-added, following the instructions here.  
    NOTE: This should not remove your existing CourseArc content placements, but please test in a testing environment before proceeding to double check. 

  2. Alternatively, the following line of CSS can be added into your theme's CSS by your Canvas administrator: 
    iframe#tool_content { width: 100% !important; }