I work with a very small team on which I am the main Programmer and Game Designer, and I can't seem to communicate effectively to the teams modelers what criteria I have for certain models for the game. We haven't been able to find anyone to do concept art for us, so we've been resorting to google image search where I try to identify major the main features of an image that I'd like to see in the model itself. But even with that, I still get models back that are borderline unusable because the design itself is flawed.

One example is a door that will be used throughout many of the games levels, in the prototype I created it as two blocks that slide apart and made sure to make it extremely wide (4 times as wide as it is tall) and the example that I gave the artists was also wider than it was tall; however the model that the artists created was more squarish. (Just as wide as it was tall). My reasoning for making to door so wide is because the player could otherwise get stuck on it very easily, so changing the design to fit the model is difficult here.

I don't blame the modelers for that kind of thing because I know we aren't giving them a whole lot to go on, but at the same time I'm not sure how to identify these possible issues before they occur (and as a programmer that drives me insane >.<). On one hand, I can't modify everything in the game to fit whatever crazy ideas the modelers come up with, but on the other I don't want to tell them that hours of their life have been wasted on a flawed design partially because I couldn't communicate the goal effectively.

I thought running through and blocking out all of the models and the basic collision for each level would be enough to relay the idea, but apparently it isn't. Another example is a mech we have in the game - the blocked version has one blaster on it with the other arm down to the side, and it only shoots from one arm. However, the modelers created one with blasters one both arms. (Which isn't that hard for them to fix, but I still am not sure where the error in communication is).

I'm currently tempted to go through the each level and model a 'concept' version of everything in blender, but I know that would almost double the estimated remaining development time. I'm just not sure what to do :/

We haven't been able to find anyone to do concept art for us

This is exactly your problem. Try drawing up simple sketches if you can't find anyone.

This type of communication often falls on the shoulders of the designer.

Designers need to document and explain the design to all the groups; the programmers, modelers, animators, audio, effects, and QA groups all need to understand what the thing is supposed to do.

Part of that understanding includes knowing why a thing needs to be that way. In this case, perhaps a single line would have prevented the problem: Doorway needs to be at least 4 units wide to accommodate pathfinding.

For doorways on other projects we've tended to ignore pathfinding and use a portal system. Doorway elements (bridges, doors, teleporters) include marked locations (like a named joint) to allow animations to hook up cleanly. The object needs to walk to the marked location with the correct orientation, play the animation set associated with the object's motion, then resume from the exit location. This allows for things like gates that need to be opened and closed, one-way portals, and other object properties.

Asset briefs are a key. I spend a good lot of time writing these up with a number of key art refs, writeups and some sketches of my own. Simple assets can sometimes takes up to 2-3 pages depending on technical constraints (including small image refs mind you).

It takes a while to write, but it is a life saver as the projects move forward, I suggest you pause and start documenting these more efficiently, and if you have such technical constraints, clearly state them, and if you must, clearly identify a 'technical constraints' box in the asset brief. You can't have the artist run around based off conversation notes.

What I will say next is probably largely untrue, but from my experience, artists tend to be less technical and more creative, which also inherently tend to make them less efficient at making mental or physical notes of discussions, making asset briefs, charts, and refs much better tools of communication.

As a designer/programmer, you should be better acquainted with data manipulation and it should be easier for you to organize that information so use this advantage to process it for them and make it intelligible. Establish a system. Create asset briefs!