building vikings?

[caravel]

Well, technically you pay the same price per man, yes. Due to the huge unit size the upkeep doubles in comparison to standard size though.

This does not equate to paying more. You pay the same upkeep per head, and you get more men for free when you train them. A 200 man units cost the same upkeep as two equivalent 100 man units.

Since your income is not affected by your choice of unit size, you may be able to pay for the same number of men but you are able to pay for only half the number of units. So, while fielding technically the same number of men (roughly), the number of units sustainable halves, resulting in smaller stacks and more 'small scale' engagements.

The number of units is not a factor. The number of men is. Also, with all due respect, it is clear that you have not played on huge units much if you think that this gives "smaller scale engagements". Battles on the huge units setting are truly epic and always have been since the days of STW, right through to the latest TW games. I would prefer 4 two hundred man units of spears to 8 one hundred man units any day of the week. All the latter has over the former is more individual units, but no more men. To summarise I don't see "units per battle as a factor" nor do I see the AI deploying less units or battles being any less epic on huge units. Also the one most important point is the units per battle limitation. Because only 16 units can take part in any one battle at any one time, regardless of the unit scale, the huge unit size clearly has more men per battle every time. Give it a try and you won't be disappointed.

Anyway, since time is an issue too, retraining becomes a valuable strategic asset. Come to think of it, it becomes even more of an exploit against the AI than it normally is already, since the AI does not retrain as far as I know.

Anything can be exploited but retraining is not the biggest exploit it's made out to be. If you want to play fair with the AI, then do not retrain and enable the "tidy up units after battle" option. This ensures that your battered units are automerged like the AI's. If you play with this off and manually merge your best high valour units together, then you are taking advantage of a bigger exploit than retraining.