Every argument for or against immigration is based on its impact on the destination country, whether that impact be economic, political or cultural. Such impact can only be measured on a per capita basis. Few people would claim that adding 5000 immigrants to the US - a country of 310,000,000 - would make a lick of difference one way or the other. But adding 5000 immigrants to the country of Liechtenstein - population 37,000 - could have a major impact on the country. For example, one argument against immigration is that immigrants "steal jobs." What is 5000 jobs against a work force of 80,000,000? And what is 5,000 jobs against a work force of say, 13,000?
Land area is irrelevant. Both the US and Canada are - relatively speaking - at low population relative to land area. Either country can easily absorb immigration in the numbers they've been taking, or even many times that number, if available space was the only issue.
BTW, unlike the US, a huge portion of Canada is at best marginally habitable due to arctic climate. Not that this matters much for immigration either because the habitable parts of either country are more than sufficient to absorb tons more immigration than either of them currently permits.