How to increase defaultStackSize in c6runlib?

You have two choices for static setting of the defaultTaskSize value of the ti.sysbios.knl.Task module In the C6run build directory, there is a file dsp_libs/os/sysbios6/C6Run.cfg which provides the base SysBios6 configuration for SysBios-based platforms.  In that file you can add the following somewhere after the "var Task" line:

Task.defaultStackSize = <insert size in bytes here>;

Alternatively, you should also be able to modify the platforms/<platform name>/sysbios6.xs file and insert both lines:

var Task = xdc.useModule('ti.sysbios.knl.Task');
Task.defaultStackSize = <insert size in bytes here>;

After one of the above changes, you would need to reconfigure for your desired platform and rebuild the back-end libraries and then rebuild any applications built using the c6run tools

The first option above would affect every SysBios platform, while the second would only affect the platform <platform name>. 

However, I suggest you try and figure out why you are running out of stack space and possibly alter your code to avoid that.  The default stack size for a task is 2KB, and unless you are allocating lots of local variables, that typically is sufficient unless you have very deep nesting of function calls.

Regards, Daniel