I agree, if at all possible it should be at least the aim to get figures in different sets compatible in size, preferably 23-25 m.m tall.
We have already touched this issue on this thread but let me take it further.
Always when the size issue is discussed, the usual reply is that humans come in different sizes. That is of course true and I have not so much issues with the human bodies being in different sizes, but big sized (and extra small) figures usually also have bigger (or smaller) weapons, bags, pouches, hats, helmets and so on. It is not the body size that bugs me that much, it's the different size of the same type of equipment.
And heads. Even if someone is 2 meters tall and another is 1,5 meters, the head (and therefore helmet/hat) is usually not that different. Of course there are a few with extra big or small sized heads (and bodies), but it is not normal to have a company composed of 50% 2 meter tall giants (unless King Friedrich's own Prussian Grenadiers :), or with 50% being almost midgets.