Always use a solid state drive - SSD.
Lack of space should be supplemented with a low cost ultra fit usb flash drive.
CPU and memory should be sufficient to allow for the use of following applications in parallel:
VOIP (voice over IP) with screen sharing and/or and screen recording. For ex: skype, hipchat, zoom.us, google hangouts, webex, slack, joinme etc.
Browsers with multiple tabs and/or sessions
Terminal or CMD prompt
Specifications should actively discourage the use of functionality which is better off on a cloud-box. Using a cloud-box sets a better precedent for how your developers perceive the deployment cycle from start to finish. This devops-first approach works in the best interest of a developer's skillset and the company's bottom line.
We recommend Apple 13 inch macbook air
roughly 1,200 USD
This still might be just a tad too powerful!
Even with these models, we haven't been able to get some of our developers to see the light: "Always build and run in docker with the cloud-box."