You need to pick a spec (console, PC, mobile, low end, high end?), get the hardware, and try stuff out. I don't honestly know of devs using existing game polycounts as a baseline. AFAIK design considerations are achieved from the experience of the artists and a fair degree of trial and error, creating the assets and then having to rejig them to fit the final profile. But you can't really choose a spec based on polycounts because those polygon counts are only how many the hardware can transform at peak. The moment you throw shaders at it, amount of renderable polygons could become anything.
It'd definitely help to know what targets other games have used, but as I say, I think devs work without that knowledge other than their prior games. One thing you could certainly do is aim for a similar level of detail. Find a game, study screenshots, and create something in the same ballpark. Ideally though, I'd suggest keeping polygons as low as you can to get your artistic look, as less to draw always results in faster updates and less variation when there's more on screen!
Also, don't underestimate impact of post effects! Run with your bloom and moblur and whatever from the beginning, because that can massively impact some platforms.
It'd definitely help to know what targets other games have used, but as I say, I think devs work without that knowledge other than their prior games. One thing you could certainly do is aim for a similar level of detail. Find a game, study screenshots, and create something in the same ballpark. Ideally though, I'd suggest keeping polygons as low as you can to get your artistic look, as less to draw always results in faster updates and less variation when there's more on screen!
Also, don't underestimate impact of post effects! Run with your bloom and moblur and whatever from the beginning, because that can massively impact some platforms.
