Because consistent sizing of the graphics and images within a document is a level of detail that will add polish to your presentations and bring them up to professional standards. Professional firms want consistency on the page, from page to page, from document to document, and from project to project. It is part of the branding for the firm; something that their audiences/clients learn to expect.
Having said that, the sizing starts at the point just after you import the framework. If you have more than one framework on a page (or even within the document) or mix and match similar frameworks, bring them in at once and size them together. Then distribute the presized images to their respective pages and start your customizations. You'll want to customize the objects consistently as well.