In terms of population France would have about 20% more people.
This is the population of OTL France 1930-1990 from the Encyclopaedia Britannica Books of the Year (I only have up to 1995 so I can't do 2000 and 2010)
1930 - 41,150,000
1940 - 41,300,000
1950 - 41,736,000
1960 - 45,684,000
1970 - 50,770,000
1980 - 53,880,000
1990 - 56,735,000
This is the combined population of Belgium, France and Luxembourg from the same source.
1930 - 49,576,000
1940 - 49,897,000
1950 - 50,671,000
1960 - 55,151,000
1970 - 60,799,000
1980 - 64,103,000
1990 - 67,377,000
AFAIK Belgium and Luxembourg had per capital Gross National Products that were the same or larger than France so Greater France is effectively 20% more powerful economically and could afford armed forces 20% larger than OTL.
The 1985 BOTY that I happen to have open says that the French Armed Forces had 492,850 active duty personnel in 1982, which ITTL would work out as about 600,000. As it happened Belgium had 94,717 active duty personnel in 1983, which is about what the increase in the French Armed Forces would have been. The Belgian Forces were 73.6% Army, 4.8% Navy and 21.6% air force with 3.4% of GNP spent on defence. For France it was 62.3% Army, 13.6% Navy, 20.1% Air Force and 4.0% Strategic Nuclear Forces with 4.2% of GNP spent on defence.