Hey Pim, A sandbox is small cloud environment, so based on what I remember it had the size of a half appengine, what should be 256MB.
The documentation of Mendix points to this page: https://world.mendix.com/display/ReleaseNotes/Mendix+DX+Release+Scheme, but it isn't available anymore by Mendix. Another page is https://www.mendix.com/application-platform-as-a-service/mendix-dx-release/ but it doesn't give you the right specs.