Modeling is the process of creating a new visual thingymabob that lowizzles around on the dohicky. Basically, the Space Marine's in the game are each one Model, and when you create a new model, you can create new terrain, units, tanks and vehicles of various types.
Once you get really good, the only limit, really, is your imagination (and the 3 dimensions limit, always a pain).
This section is dedicated to a collection of information and tutorials on how to get a model into Dawn of War. You are not likely to find tutorials on how to model, although you'll have to use 
3D Studio Max for modeling.
The latest (1.11) patch notes note that:
I'm lead to belive that GMax, the free 3D studio max alternative, is no longer supported. I find this a real shame, as most modders won't have a casual $3, 495 on them. I hope GMax is updated, or perhaps Relic will do something about it.
It needs the newer versions for fixes and updates for the Expansion(s)
It is a good idea to use the latest mod tools, which include fixes to previous problems with modeling.
Here, until some overviews are created on modeling, is some unorganised information specific to Dawn of War.
new Modeler/skinner : Questions about Poly LimitThere is no actual upper limit for polygons in Dawn of War. Here is a list of models in the game and thier poly count:
Exisiting Polycounts in dow:
Some additional information on thse numbers:
Ok i don't know an actual limit. But for reference the Space Marine unit is around 2000 poly's so thats a good point of reference. These would be in a squad of up to 9. So it's possible you could go higher on something like say a hero unit as it's not going to be in a squad.
Just a quick side note... When talking poly's, its best to use tri's, as the meshes exported from Max are triangulated. So the marine.ref file had about 3500 tris
There is no (good) way to get models from the game and remodel them – apart from using the source models (in the LARGER of the two possible mod tool downloads on the net), consisting of:
These include animations. Textures can be got from the game resources for any model, and reskinning is an easy way to make a unique army.
Tutorials? on Modeling in Dawn of War: