Hi Young,
customization of the image is done through the Yocto buildsystem. If you aim for a small root filesystem, then "imx-image-full" is probably not a good starting point.
The Yocto refernece documentation lists the relevant image recipes:
https://www.nxp.com/docs/en/user-guide/IMX_YOCTO_PROJECT_USERS_GUIDE.pdf
As you do not tell us a lot about your requirements, I can only speculate that maybe "core-image-minimal" or "core-image-base" may be better starting points for you.
In the end you should aim to customize the provided Yocto environment for your own board so that you can build the exact software that you want to distribute. A short introduction on how to do this can be found here:
https://www.nxp.com/docs/en/user-guide/IMX_YOCTO_PROJECT_USERS_GUIDE.pdf
In the end I think you will need to read up a little bit more about Yocto itself, which is a common build system for many SoC vendors nowadays. Most of this is not specific to the NXP distribution itself. Please expect that this will require some time to do correctly.
