The reason he asked "when," is because there is a break in the middle of the 1813 campaign – the armistice – during which the French in particular, but all sides in general, greatly increased their cavalry strengths.
So if you were doing a scenario from the Spring campaign, entire French cavalry regiments might have as few as 120 men each. "Squadrons" might exist in name only.
Whereas if you were doing the Autumn campaign, they might be two or three times that size.
And then, of course, things are different with the Guard
and different yet again for those French cavalry regiments that were brought out of Spain and arrived in the Autumn. The French Guard cavalry was dramatically increased in size during this campaign, although often augmented by "Young Guard Squadrons" of dubious value. Still, the Guard cavalry more than doubled in 1813, and you can find regiments with upwards of 1000 men.
So if you want just a totally fictional number that will get you in the ballpark
French line cavalry regiments in the Spring 1813: #2-4 squadrons, totalling @100-200 men.
Actual OBs are never as clear or simple as wargames need them to be. As long as you represent the French line about 80% understrength
the French Guard about 80% overstrength
then you'll get the picture.
Prussian and Russian regiments are probably closer to about 400-600 men each, in the Spring. If you're doing Leipzig, the Austrian regiments in the Autumn campaign are significantly larger.
* Just as one interesting example, in early 1813 Bourdesoulle's Cuirassier "division" comprised:
2nd Cuirassiers (250 men with 81 horses)
3rd Cuirassiers (135 men with 13 horses!!)
9th Cuirassiers (305 men with 13 horses)
6th Cuirassiers (159 men with 37 horses)
11th Cuirassiers (257 men with 131 horses)
12th Cuirassiers (121 men with 21 horses)
The total number of mounted men in this entire division? Not even 300.
By early September, these same six regiments could put about 1,300 men in the saddle, all-combined. I'm not sure what their strength was six weeks later at Leipzig, but surely no more than that.