A key points to keep in mind about tag wikis is that a tag wiki is not an encyclopedia article. A tag wiki should provide information that is relevant to your community. This especially goes for excerpts, which should be brief so that you can find out the meaning of a tag at a glance.
Although you can have up to 460 characters in an excerpt, most excerpts should have far less than that: the character limit is for complicated tags where for example you need to explain that questions about some topic that may look very similar should actually use a different tag. A good tag excerpt is as short as possible, not as long as possible.
So, for example, do not define “Canada” by saying how many provinces it contains. At the scale of an excerpt, it's completely irrelevant. In the tag wiki body, mentioning provinces and territories may be relevant to state which kinds of laws are issued at the federal vs provincial/territorial level. In the tag wiki body, you may want to link the tags corresponding to the provinces/territories for convenience.
A word like “Canada” probably doesn't really need a definition, so writing an excerpt for it. However you may want to provide generic guidance about countries, such as “the laws and regulations of Canada and its provinces and territories, and international treaties that Canada is a signatory to”. (Assuming that's the tagging policy — if questions about a specific province's laws should not have the [canada] tag, word the excerpt accordingly.) You should hold a discussion on Meta.Law to standardize on a format for country tags, with maybe some variation depending on the level of devolution.
The point is that Canada doesn't need to be defined. But the exact meaning of the tag in your community might.
For an obscure concept, a short definition is warranted. This doesn't need to be, and usually shouldn't be, a precise definition, just enough to provide some context and avoid confusion from readers who aren't familiar with the terminology. For example, the excerpt for [common-law] currently reads
For questions regarding English common law, or legal systems based on judge-made law descended from it.
“Common law” doesn't exist in many legal systems, so people who visit Law.SE from non-Anglo-Saxon countries might not be familiar with it. So having this bit about “judge-made law” is useful.
A tag like [florida] would be an intermediate case. The excerpt and body should include the mention that Florida is a US state, but not define it ambiguously by detailing its geographical location inside the US. The tag [georgia] needs a definition to indicate whether it's the country or the US state — either something like
The US state of Georgia. For the laws of the country called Georgia, use the tag ???.
Georgia is a country in the Caucasus. For questions about the US state, use the tag [georgia-state].
plus some indication of when to use [united-states] in addition. For the tag [new-york], indicate whether it's the state or the city and how to tag the other meaning.
And indeed don't prefix the excerpt with “For questions relating to …”, that's just noise. Keep in mind that people's attention drops very fast when they're in scanning mode (which is the case for excerpts, much less so for wiki bodies). Every word counts at the beginning.