Now Available: Repent or Die: Great Battles of the Renaissance, Volume I

Actual strength of units is not well known for this period. Often the overall strength of both armies can vary depending on which source you use. here are exceptions for specific battles. Breitenfeld is one where unit strengths are known with cavalry units ranging from as few as 100/200 men per unit to 800 men per unit. Infantry units also varied. Rather than using different sized units I averaged the strengths of each unit. The was partly because I use 6mm figures on 80mm bases (I had based them for another rule set years ago that also does not use specific unit strengths and did not want to rebase them), I use an average of 400 men per cavalry unit, 600 men for a "battalion" of pike and shot, and don't really care how many men are in each tercio. Since Repent of Die uses unit cohesion/morale instead of counting the number of figures/men lost actual unit strength does not matter. The battles in the scenario book can be used for any other rule set using the same average strength per unit that I use.
As for actual strengths of historical units. The early Tercios had a paper strength of 3000 men though campaigning usually resulted in lower strengths. By the time of Breitenfeld average strength of a Tercio was about 1500 men. The Swedish Brigades also varied in strength and by the time musketeers were detached to assist the cavalry units they averaged about 1700 men in three "battalions". The 800 man cavalry units were often split into two units during the battle instead of remaining in one large hard to maneuver unit.