Hi Stephen,
Typically if your projects are organized in a hierarchy with a parent project you could run the report from that top most parent which would show all of the data of the child projects as long as you have enabled the ability to view items from child projects.
You can check this by clicking the project name in the upper left to access your project list. At the bottom of the list of projects there is a section labeled "Show Items From" with two options, be sure to enable the Child Projects option. Then make sure to select your highest level parent project. Afterward you should be able to run your report.
Unfortunately I don't believe there is a simple way to run a report through the UI for the entire workspace if your projects are not organized into a project hierarchy with a top level parent.