What would be ideal now? Are 28w Infantry divisions really better than 40w? What's the ideal mix of INF to Art in a template? What's the ideal mix of Tanks to Motorized in a Tank division?
Let's look at attack, defense and mixed.
Attack: Wide divisions still take less losses ... but medium and narrow are more likely to win, since they have more total organisation present. As you're a human, your attackdivisions are unlikely to ever be in a losing battle, so go wide.
How wide? Well, if you know ahead of time which terrain the crucial battle will take place in, you can *really* optimise width. If not, it seems 42-45w give more stats on average, especially if you can trigger the encirclement tactic (name?). (Personally, I'd go for 38w because I like cost-efficiency to a fault.)
As for composition, it's the same as always: More foot/mtd/mot/mech infantry gives more org, allowing you to fight longer, thus increasing your chance of winning. It also gives more HP, which, afaik, lessens material losses. On the other hand, more tanks/art means more dmg and a shorter fight ... as long as you don't run out of org. 50%-50% seems common these days, though I think up to 75%-25% sees some use too, though only for larger divions.
Are flame-tanks a recent addition? Certainly use them. Also, signaling is supposed to help with coordination: divisions now spread their attack around, but coordination from tech and Signal Support is supposed to help them foucs somewhat on one target (like 50% targetted damage and 50% spread evenly).
(@beginners: Be sure to include maintenance and flame-tanks in all tank divisions. Often you will want logistics too, since tanks tend to concentrate, thus exhausting supply locally.)
Defense: Usually, defenders are not overpowering their enemy, so stamina matters a lot more. 15-22w is common, but the lower ones only with air-superiority and plenty of CAS.
Composition-wise, 7 inf to 2 art maximises dmg done before breaking, but support artillery will change that balance. 1 Line Artillery + 6-9 Infantry, supported by Eng + SuppArt + SuppRockArt should be close to optimal (though equipping both a flame-tank and scout-tank with dozer-blades is fun

).
Mixed: I'd go for around 25-26w for divions that may end up in either role, such as dragoons following behind tanks and inf-for-mopping-up-kettles, though I suspect others will go up to 28w. I've heard 25 is a local optimum for a weighed average.