Yes, the Gmail signature is restricted to 10 000 characters in code.
Don't forget that the synchronized signature will very likely have fewer characters than the template since it doesn't contain conditions and other "assistive" code elements.
We monitor the size of your templates to make sure you don't exceed the limit. If that were to happen, you will be notified upon saving it. It is also possible the limit will be reached only for specific users upon set-up; in that case, the process will end up in error, you will be notified and not charged for the users that ended up in error.
If you are struggling with the limit, we will be happy to help you optimize your signature code.