Size does not matter as China, the country with the largest population in the world with over 1.3 billion people has not qualified for Russia 2018 while Iceland with the only population of approximately 335,000 has qualified for the Mundial with a 2-0 win over Kosovo.
See the list of the smallest countries (by population) that have qualified for the World in its history.
Country Estimated Population at time of World Cup Qualification
Trinidad & Tobago 1,300,000
Northern Ireland 1,850,000