Hi Ashley, you can certainly adjust the image sizes however you want. If you look in single.php we just use the_post_thumbnail() to display the image.
You can either pass in default parameters for sizing, i.e. ‘thumbnail’, ‘medium’, ‘large’, ‘full’. Or, you can add your own sizes via the add_image_size() function in WordPress and then use this size as a parameter.
More details on image sizing in WordPress can be found at: http://codex.wordpress.org/Function_Reference/the_post_thumbnail
Note, if you’re going to modify single.php make sure to do within an ‘events’ folder in your theme so as not to lose your changes when updating.