Hey Jack
The problem you face with Cassano, and indeed for almost any WOSS battle, relates to the availability of records. The OOBs in Nafziger, and books on the war, are almost entirely drawn from 2 published collections:
1. ‘Mémoires militaires relatifs à la succession d'Espagne sous Louis XIV’ published by the French Army between the 1820s and 1860s
2. ‘Feldzüge des Prinzen Eugen von Savoyen. Series 2’ published by the Austro-Hungarian Army between the 1870s and 1890s.
These are both multi-volume works containing narrative, some analysis, and also a great deal of primary source material in the form of contemporary correspondence, reports and OOBs. They are a superb resource, but have some limitations. They both occasionally include strength returns, but these are quite rare, and are often just establishment figures rather than actual strengths. From memory there is nothing in either which would give strength returns for this particular battle.
There are masses of additional primary documents in the French army archives at Vincennes, and in the Austrian Kriegsarchiv in Vienna (from which most of the stuff in the above sources was drawn). I do not know this for certain, but think it’s unlikely that strength returns are available for Cassano, as full army musters on campaign were irregular. Even if there were ones, there would be reason to doubt their accuracy, because of the contemporary army structures, poor methods of recording, unreliability and venality of those involved in the counting.
Because of this, generals of the time tended to estimate army strength by counting the number of infantry battalions and cavalry squadrons available to both sides. Their correspondence does not usually quote numbers of troops. That said, when making their plans, they did take account of the strength/quality of their units versus those of the enemy army, if there was known to be a significant difference in that regard. Where modern historians quote army strengths, this is done on the basis of multiplying the numbers of battalions and squadrons by assumed average unit sizes, taking into account establishment strengths and some estimate of attrition to date in the current year. Such an approach is fraught with difficulty, but it is the one I believe you must follow.
You say you have army sizes. Frankly, I would start by questioning those, as many modern books on the war simply re-tread other secondary sources, and any figures will only be as good as the assumptions made. And of course these do not give you the sizes of individual units. You really are going to need to guesstimate those, based on such factors as establishment strength, strength at the start of the campaign (note the two are unlikely to be the same, particularly for French forces in Italy), and likely attrition before Cassano. You will at least have the comfort of knowing that no-one else will be able to easily then shoot you down!
Sorry to be the bearer of bad tidings, but am happy to assist if you do decide to persist with your madcap adventure

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Martin