There are a few ways to classify the size of a town, but the most usual (and probably the most effective) is to use the population as measured in the most recent census of the country.
Hence the schedule below represents the 'most populated' biggest French towns and has been prepared on this basis. The list is in descending order and uses data from 2011. The populations have changed slightly since, but the ranking according to size remains unchanged.
We also have an article highlighting the most visited places in France if that is what you are looking for.
1. Paris
Population: 2.25 million
Region of France: Paris Ile-de-France (105.4 square kilometres)
See the Paris guide for details
2. Marseille
Population: 851 thousand (240.6 square kilometres)
Region of France: Provence
See the Marseille guide for details
3. Lyon
Population: 491 thousand (47.87 square kilometres)
Region of France: Rhone-Alpes
See the Lyon guide for details
5. Nice
Population: 344 thousand (71.9 square kilometres)
Region of France: Provence
See the Nice guide for details
6. Nantes
Population: 288 thousand (65.2 square kilometres)
Region of France: Pays-de-la-Loire
See the Nantes guide for details
7. Strasbourg
Population: 272 thousand (78.3 square kilometres)
Region of France: Alsace
See the Strasbourg guide for details
8. Montpellier
Population: 265 thousand (56.9 square kilometres)
Region of France: Languedoc-Roussillon
See the Montpellier guide for details
9. Bordeaux
Population: 239 thousand (49.4 square kilometres)
Region of France: Aquitaine
See the Bordeaux guide for details
10. Lille
Population: 228 thousand (39.5 square kilometres)
Region of France: Nord-Pas-de-Calais
See the Lille guide for details