There are some apps to help with customizing the experience based on their country:
And you can look through the shopify apps:
If the shopper’s country isn’t one that you have a website for (and thus cannot auto-detect it and redirect to it), Reserved’s country selection screen could be a good option. But I’d recommend mentioning something to the effect that you noticed they appear to be in Canada, and that you don’t have a Canadian store – and to please choose a country from the list below. My hunch is that Reserved does not auto-detect your location (and they should).
As for measurement units, the US uses the imperial system, whereas the rest of the world uses the metric system. So the units should be adjusted based on the location of the user.