Extractors don’t always work well with seized bolts. If the bolt won’t turn and breaks off, an extractor of smaller diameter will probably break also. With some skill drilling the broken bolt out with the right size drill bit so that just the threads are left behind is sometimes the only way to remove a seized bolt. Then clean the bolt threads from the hole with a pick or sharp punch then a tap.
The best way to approach this is to take every precaution not to break the bolt.