There is no compact, reliable way to determine the size of an AVIF image. A standalone C snippet called libavifinfo was created to partially parse an AVIF payload and to extract the width, height, bit depth and channel count without depending on the full libavif library.
avifinfo.h, avifinfo.c, LICENSE and PATENTS were copied verbatim from:
https://aomedia.googlesource.com/libavifinfo/+/2b924defa4c2cd227540efe164067a8cc913eeba
They can easily be kept up-to-date the same way.