(state senator not U.S. senator)
Just from looking at the map for Montana I'm guess population still. Some Senate distracts are only 1 county, some are 3 or 4. Like Flathead County has 3 or 4 different Senate districts in it. But Senate District 9 is made up of (at least parts of ) Teton, Lewis and Clark, Powell, and Cascade Counties. http://funadvice.com/r/begot8rvnns
Now I don't know if every state follows the same rules or if each state has a different way to of doing it.
There's 100 senators, 2 for each state. So it depends on which state they're in? California is much larger than Rhode Island. Perhaps you're talking about representatives? And that is also dependent on population not land area. States are divided into congressional districts based on population. Further details will have to come from an American.
I believe its determined by population not my acreage. But I don't know what amount of people 1 state senator represents, and it very well could very from state to state.
i thought it was 1 to 3 counties