I have tried to make the editor wider, but the sidebar width is currently limited by the Google Docs add-on infrastructure to 300 pixels. Possible work-arounds:
Use the zoom feature of your browser. For example, in Chrome 47 on my HD monitor, I can zoom to 175% before the sidebar starts taking space from the main editor.
Use PlantText.com as an intermediate editor, manually copying/pasting the PlantUML source back to PlantUML Gizmo.
Yes. Select the PlantUML diagram you inserted and click Edit selected. The source code should appear in the PlantUML source text box. You can change the source, and click Insert again to update the diagram.
There are a few limitations, however:
You should not change an image's layout from In Line to Wrap Text or Break Text, because Google Docs™ silently removes the image's Link, which is where the source is stored.
You also should not modify the inserted image's link yourself.
If you change the PlantUML server in the settings, any images generated by a different server will not be recognized.
The source code is "zipped" (encoded) into a URL to make an image on PlantUML.com, which the PlantUML server uses to draw the diagram. PlantUML Gizmo inserts the diagram into Google Docs™ or Google Slides™ and attaches a hyperlink (the same URL that was used to draw the diagram). PlantUML Gizmo uses the attached hyperlink to "unzip" (decode) the source code in the sidebar editor when you select a diagram and click the Edit button. There is more technical information about zipping and unzipping from Javascript.
PlantUML Gizmo uses PlantUML.com to render its diagrams inside Google Docs™ or Google Slides™, so you don't need to install any extra software.
The best way is by examples. PlantUML Gizmo has a menu of sample diagrams. You can find more samples in the PlantText UML Editor and on PlantUML.com.
If you don't want PlantUML diagrams to be drawn on the public PlantUML.com server (because, for instance, the diagrams are confidential), you can use a different PlantUML server (e.g., one running on localhost).
To specify a different server, go to PlantUML Gizmo > Settings.
If you want to use a server running on a private network (e.g., localhost), you must:
specify the Settings option PlantUML server is on private network
run the version of the PlantUML server later than 20 March 2018 that supports "data:..." URIs for images and CORS.
configure the private web server to allow CORS -- however, the default configuration of the embedded jetty PlantUML server already allows this via the command: mvn jetty:run that normally means the server's URL is http://localhost:8080/plantuml/
Yes, and it simply involves using the !includeurl feature of PlantUML. For example, in the PlantUML source of PlantUML Gizmo, use the following:
!includeurl https://raw.githubusercontent.com/fuhrmanator/course-activity-planner/master/ooad/domain-model.puml
Read more about this on Stack Overflow.
An interesting tool that inspired parts of PlantUML Gizmo is the PlantText UML Editor.
PlantUML itself is a very active project, with more functionality being added often. Check PlantUML.com for more information.
I use PlantUML in several of my software analysis and design courses (in French) at the École de technologie supérieure (ÉTS) in Montreal.