Update (25 Oct 2020): With Winter'21 release,
enabling org shape has been released to all environments. Although still in beta, it facilitates experimentation with org shape creation and easier scratch org creation as needed.
At the moment, I'm not aware of any tool available for free to help you draft the scratch org definition file. You may want to check whether paid tools like Gearset, Autorabit etc. has something to offer in this reference.
In lack of appropriate tool, you could start building the definition file from scratch (which might most likely be a mundane & time consuming task). For some help documentation, refer here & here.
- If you have some documentation and version control for the existing prod org, it might help you narrow down to the list of required features/ settings.
- Take precaution not to include every feature or setting into the scratch org definition (as some of them might be critical, confidential or meant for prod only) and run into errors (or performance degradation) with scratch orgs.
SFDX CLI command
force:org:shape:* is meant to solve this problem (which is still in beta) and in order to test this out, you need to have
Org Shape feature enabled in your org. I think the
Org Shape feature is enabled as a pilot for very few orgs (so, you might want to check if your production org was signed up for this). Check out this youtube video, this latest release notes & this section of SFDX dev guide. You may want to join this community to participate in discussion and track some updates.