To add a new size context, you go to http://dev.bc.telavant.com/admin/structure/scald and scroll down... click "add context".
The actual context is just a name at this point, so like I have "thumbnail" "small" "medium" large", etc.... just name your size context accordingly...

Then, click on "context" next to Image or whichever file type you are going to edit and assign the new Context for the Image to http://dev.bc.telavant.com/admin/structure/scald/image/contexts

The new Context you created "small" large "medium", etc. should appear in this list

Just toggle open the one you want to edit and add a size constraint

It's Under "transcoder"

More on contexts:   https://www.drupal.org/node/2104651

You should be able to use the Thumbnail, Medium and other options I have there as examples

Now... the last thing you'll need to do... on http://dev.bc.telavant.com/admin/config/media/image-styles

you can add new sizes, new scale options, cropping options, etc. on ^^^ that page

Drupal which has the Image Styles (http://dev.bc.telavant.com/admin/config/media/image-styles) and Scald provides an interface

More info:
http://heim.ifi.uio.no/gisle/staging2/drupalprimer/drupal/mod_scald.html

CSS info:
http://drupal.stackexchange.com/questions/150352/adding-a-class-to-scal…