Hi ChefBoyRDee,
Fortunately, no large non-technical battlefield effect should be felt when changing the unit sizes. Some have found that cavalry charges are slightly more deadly to infantry on smaller sizes though, but I haven't really seen this myself.
In R:TW and M2:TW, on the strategic map, the larger units drain your cities populations at a faster rate. I like this feature since it can be used to keep squalor down, but it may also force the player to consider the importance of city growth in higher regard - especially if they have many small settlements.
On the technical side of things, upping the unit sizes can create battlefield performance hits on slower computers. This can result in large amounts of lag and the game sometimes slowing to a standstill on larger battles. Of course, this does work the other way, and decreasing the unit size may provide you with performance benefits.
Hope this helps, good luck, cheers!
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