Hello WilliamCwo,
Thanks for reaching out in Apple Support Communities. I understand the size of the stickers don't appear to be correct and allow the behavior of the stickers is odd. I'm happy to help.
Try closing the message app and restarting the iPhone: Restart your iPhone, iPad, or iPod touch
If this issue only happens with this app, try deleting the app and adding it back to your iPhone:
How to delete apps on your iPhone, iPad, and iPod touch
Follow the section of this article that states "Manage iMessage apps" to add the app back to your iMessage drawer: Use iMessage apps on your iPhone, iPad, and iPod touch
If the issue only occurs in this particular app, and persists, contact the app developer: Contact a third-party vendor
Best Regards.