Although you may use multiple mesh refinement controls with various levels of refinement with respect to the global mesh size, the automatic meshing solver has a limit on the depth of mesh refinement achievable given the bounds of a particular geometry. Here is the rule to determine the smallest mesh size allowed:

Take the largest edge of the box bounding the geometry (if there is a stroke in the engine, consider it to be fully extended). Divide it by 16384. The resulted number is the smallest mesh size allowed.

If this limit of smallest mesh size is breached, you might see an error message in the log window of Forte Simulate User Interface like this:

Error: refinement control depth of 1/32 in refinement control *** cannot be obtained with the current geometry bounds
Error setting up the domain, exiting with status -1

Or, the error message might show up in the MONITOR file in the working directory like this:

****
forte: OctreeMeshController.cpp:2649: void OctreeMeshController::postUpdate(Domain &, double): Assertion `flags.size() == (size_t) vm.size()' failed.
forrtl: error (76): Abort trap signal
****

In either case, consider reducing geometric size of the problem and/or relaxing the mesh refinement depths.

Related:

If mesh generation takes unexpectedly long time, one of the reasons is that the requested mesh size is too small. This is discussed in Mesh Generation or Mesh Preview Takes a Very Long Time.

In internal combustion engines, the settings related to valve activation usually affect the smallest mesh size. They are discussed in Movement Type.