Caveat: I am not an expert, but an fascinated by the question too and pulled some stats off Wikipedia for very large/largest battles in various wars. Note that this is extremely Western centric, and there are soooo many different other wars and batles that could have been included. The point is to see if there are and trends in survival rates in battles on a battlefield.
1943 - Battle of Kursk (49 days)
Axis: 780,900/940,900, c252,000 casualties USSR: 1.9m/2.5m, c980,000 casualties
1916: Battle of the Somme (c138 days)
Allied: c250,000/c1,100,000, 623,907 casucalties Entente: c120,000/c600,000, c237,000-500,000
1870: Battle of Gravelotte (1 day):
Prussia: 188,332, ~20,000 casualties France: 112800, ~11,500 casualties
1863: Battle of Gettysburg (3 days):
Union: 93,921, 23,055 casualties Confederacy: 71,699, 23,231 casualties
1813: Battle of Leipzig (4 days):
French Empire: 225,000/155,000, 120,000 casualties Coalition: 380,000/430,000, 54,000 casualties
1758: Battle of Zorndorf (1 day)
Prussia: 36,000, 12,000 casualties Russia: 43,500, 18,000 casualties
1704: Battle of Blenheim (1 day)
Allies: 52,000, c12,500 casualties France: 56,000, c34,000 casualties
1632: Battle of Lutzen (1 day)
Sweden: 19,000, 5,000 casualties HRE: 22,000, c5,000 casualties'
1347: Battle of Crecy (1 day)
England: c10,000, 300 casualties France: c30,000, c2,000 casualties
etc as I'm tired. The earlier time you will see large armies of more than 10,000 men or so is with hte large empires, such as the Byzantine, Persian, and Roman empires.
Just based on the data from the battles above:
I think the major takeaway is that post-industrial battles (of the 20th century) tend to be safer for combatants on a day to day basis than earlier battles. However, if you consider that early gunpowder armies might fight two or more battles (plus skirmishes, raids, security detail etc) in the time it takes for one modern day battle to play out I would choose to be fighting in the modern period. Most of the battles above seem to be among reasonably matched opponents, although the Russians were wasteful with the lives of its men in the early-mid WW2 years.