Is this tax sized to cover for that externalities or the amount is decided in other basis (likely to maximize government revenue)?

Is the collected money exclusively used to counter the externalities or is the spending left to the discretion of the government?

I'm asking from ignorance, but if the answer of any of those questions is no then the purpose of the tax is not to make up for the externalities.

> Is this tax sized to cover for that externalities

No, nothing like it. The final costs to the state of excessive sugar consumption are _vast_. This levy is primarily to discourage people from consuming loads of sugar.

> Is the collected money exclusively used to counter the externalities

While I don't know, I'd be very surprised. That sort of bucketed approach to tax collection/use is _extremely_ inefficient; the only place it can possibly be justified is social insurance.