Using 10.2.1 of MCUxpresso, if the 'Enable Parallel Build' setting in C/C++ Build/Behavior tab is set, the post-build step in C/C++ Build/Settings does not execute.
Is this an Eclipse issue or an MCUxpresso issue?
Seems to be working here. By default the post-build step will typically be used by all projects to run "arm-none-eabi-size" on the the generated image.
One problem could be if you have edited the post-build steps and left a # character at the start of the "wrong line". See section 18.13, "Post-build (and Pre-build) steps" of the MCUXpresso IDE v10.2 User Guide for more details.
If you are still having problems can you please provide more details of your project, its target MCU, and its configuration?
MCUXpresso IDE Support