paul-frost

This method requires the CE Image addon.
Tell your editors to change the width/height/alignment in the image properties pop up to 300px or whatever and set the alignment if they want, but the actual size displayed on the page will be decided by you in the template code and CSS.

In the EE template:

{exp:ce_img:bulk max="500" crop="no" allow_scale_larger="no" quality="80" output='<a href="{orig}" class="embeded_image" title="{attr:title}" alt="{attr:alt}" style="{attr:style}"><img src="{made}" /></a>'}
  {wygwam_field}
{/exp:ce_img:bulk}

In the CSS: