During your 2nd year, you and your class list your top 5 choices for rotations for 3rd year and you are placed there in a randomized lottery system. Some of the sites are year long sites where you'll do all your rotations at the same hospital. Other sites might have you moving here and there but you'll roughly be in the same geographic region. If for some instance you do have to move they try to minimize it by scheduling your rotations adequately. For me I stayed in central Cal for a 5 month chunk and was in the bay for the rest of the year rather than switching back and forth every rotation so it wasn't too bad.
For 4th year, you set up rotations all on your own and this is true at most DO schools in the nation since we don't have an affiliated medical center. Generally there's more moving around since you'll be doing audition and sub internship rotations in the earlier part of the year before residency apps are submitted however if location is a huge concern for you due to family or significant other you can choose to do everything in one location. The key is to just plan in advance.
There's definitely certain rotations that are more popular. Each site accommodates a different number of students. Some of our larger sites can take around 20 students and our smaller ones can take 5 students. I'd say most of my class got within their top 2 choices for 3rd year rotations so it wasn't too big of a deal. This probably just depends on your class preferences every year.