As a default if different canvas sizes are used in a ME product the customer would have to start the editor, select 'Menu', click on the current canvas size, and then select a different canvas size if they wanted it, but here are a few easy steps so the different canvas size options show on the product landing page as a dropdown list.
First, you need to create all the different canvas sizes as separate sizes

Second, you need to assign all these canvases to your ME product
Don't forget to 'Save changes' when done

Finally, you need to enable a setting in 'MegaEdit Products Settings' > 'Product Attributes' Tab > Enable 'Show Canvas Options on Product Landing Page' > 'Save'

Now all your canvas sizes will show on the product landing page as a dropdown.
When any of the options are selected and then the editor is started the correct canvas size will load.

As you can see 'Default' is also showing for all the canvas sizes.
This is the 'Canvas Sizes' name and can be edited to show whatever you want. (See the 'Canvas Sizes' tab inside each of the newly created canvases)
Just scroll to the end of the section and click 'Edit' > Make the change > click 'Update'

For example:

Please note that you can not save the name as blank/nothing, however, you can just use a space (so it appears as blank/nothing), but that will still leave the dash showing.
And here are the results of those changes:
