You can find string options in Render Setup > Performance > Development/experimental stuff. They work the same as .conf files, and can be used to set additional parameters, which do not have direct GUI controls.
Available string options:
string system.sceneExportPath = "C:/scene.scn"
Exports the scene to C:/. Rendering with this option has the same effect as clicking “Export scene…” button and selecting the same path
int dr.numThreads = -1
Makes the Corona DR slaves use all but single thread. The number of threads used on slaves is a different parameter from the regular number of threads, so by default all slaves use all available CPU cores. If you for some reason need to change this, you can use this option to set up the number of threads manually. Same semantics as for regular number of threads apply (0 is unlimited, positive numbers are number of cores used, negative numbers are numbers of cores left idle).
float mtlEditor.lightsSize = 1.0
Changes light size in the custom material editor preview. Default value: 1.0.
float mtlEditor.lightsIntensity = 1.0
Changes light intensity in the custom material editor preview. Default value: 1.0.
int renderStamp.fontSize = 8
Changes the font size of VFB render stamp. Default value is 8.
int system.threadPriority = Low
Sets the Corona worker threads priority. This is the replacement of the removed "low thread priority" switch. Allowed values are Low, BelowNormal, Normal, AboveNormal, High. Default value is Low.
bool shading.enableAa = false
Disables antialiasing for the beauty pass, and all other render elements.
float geometry.displace.maxSizeScreenOutFrustumMultiplier = 1
Allows displacement to be calculated outside of the camera view. This is useful if displaced objects are reflected in other objects, or if you are using caustics and they are producing incorrect results.
Warning: this can dramatically increase RAM usage!
See: Caustics appear cut off / with artifacts!