You think you have a choice? Just be prepared for being disappointed. But the worse off you are, most needy, most protected and most in need of accommodations, you will have priority. If you have a medical disorder be honest about your needs.
To be fair, it’s better than the questions from incoming 10th graders to Andover who ask which dorm to choose, where the answer is that they have their choice of dorms that the current 9th graders did not want. 
I’m surprised there are choices st some schools and that the quality of the rooms vary so much.
Don’t let @Golfgr8 scare you. Not all dorm rooms are terrible.
At Thacher, there is no choice, but the freshmen dorm rooms are all quite nice, especially in the girls dorm. We love the high ceilings and skylights that make the otherwise tiny single rooms feel incredibly spacious … lots of natural light, too!
At Chatham Hall, you earn room lottery points by keeping your room neat. If you want priority in picking a room sophomore year, you just to keep your room spotless.
This varies by school. But at Andover, there are about 40 dorms, built at different times, many of which are converted homes. So aside from the fact that renovations are not all done at once, many of the layouts are unique. I found the same when I went to college.
At schools where dorms are basically cookie cutters and the doubles are a standard size with halves being mirror images, it’s less of an issue (other than perhaps distance from class buildings). But then those dorms also have less character.
My experience and my kids - in college you will look back fondly on your boarding school dorm rooms and realize how lucky you had it!
I’ve actually heard from new lowers that they tend to get slightly better dorms than the current ninth graders at Andover due to how the housing lottery works.
Depends on your school…very few of the NE schools have new dorms. Some schools have dorms dedicated for Freshmen and/or by Uppers/Lowers. In general, at least in the NE, the best dorms/rooms are not given to Freshmen. There are a few schools where students of ALL grades live in the same dorm. Keep in mind that most of the NE schools frequently mentioned on this site are OLD. Many used to be single-sex schools and had different set ups. Also, different needs of students years ago. On our revisits, Hotchkiss seemed to have the relatively newest & nicest dorm for Freshmen. The biggest rooms with walk-in closets we saw were at Miss Porter’s. Most Hogwarts themed were at Emma - some with fireplaces and gargoyles outside the windows. I hear Westover has large rooms with windows seats in some.
To the OP’s original question @ how to choose? When you do have a choice of dorm, think about the location that serves you best. Do you want to be in the middle of all the action? Do you want a dorm close to the athletic center or, instead, the library? Do you need to be close to the pool or ice rink? Some students like to be near the dining hall or chapel (if there is a morning check in). Think about which dorm you would like to be residing in when it’s January (miserable) and you’re walking in the snow.
After you thought about this, go out to your parent’s car with your sleeping bag and spend the night in the back seat. That is about how much personal space you will get in some dorm rooms (LOL). Chances are…You will have a nicer dorm Freshmen year at BS compared to your Freshman year at college.
A small counterpoint that will no doubt sound crumudgeonly…: learning to co-exist with a roommate is a really important life skill. I would recommend that, absent compelling health reasons, new BSers roll the dice and receive a randomly assigned roommate if that is an option for one’s first year.
Is that stressful? No doubt. Might there be conflicts or bad matches? Surely. Is it easier to live alone? Absolutely. But learning to peacefully, respectfully and cooperatively coexist with someone who is different than you is not only a skill that will come in handy in ways that are difficult to anticipate, but also opens up the opportunity for a unique, strong relationship in a way that is a bit different than that developed by being dormmates or classmates.
Wow, finding all this after I’ve filled out my housing survey is nerve-wracking! Now I’m really anxious about my dorm! 
@RedLioness I can assure you it will all work out fine. Really there are no bad dorms and since new kids move in every year, the culture of each dorm is dynamic. The worst experiences we had with our 4 kids dorms has always been when we tried to over control the situation ( pushing for specific dorms, roommates etc…).
Yeah, that makes sense. I’ll try not to worry.
Do you mind elaborating on those dorm situations, though? I’m curious!
anyone know anything about the choate library dorms?
I highly recommend having a single, both in college and a boarding school. I have a single currently and I have practically turned into a smart room with all my tech in it. It’s nice to have somewhere to reflect in silence and have the privacy of a single.
@MrThatcher did you attend prep school?
Some students find having a roommate is not a bad thing…and very often the costs of a single are higher. Some students just can’t afford that option. It seems your family has a bottomless pit of money for you…but that’s not the case for everyone.
The OP asked the question in April. At this point, housing decisions have been made, although not necessarily communicated to the students.
For most boarding schools, at least the ones most often mentioned on this site, the cost of boarding is the same is the student is in a single or a double.
@skieurope It can be argued that it’s still valuable, as the kids from the next application cycle can have some wisdom regarding the benefits and costs of sigles and doubles.
@thumper1 what school makes you pay more for a single rather than a double? That sounds awful!
It can be further argued that kids from the next application cycle will not read this thread and will ask the question anew. Case in point - this is far from the first time this question has been asked.
None of the boarding schools discussed frequently on this board charge differently for their various room configurations. Singles, doubles, triples, etc. all the same rate. Tiered pricing for rooms is a college thing.
@skieurope that’s a fair point. Well played.
@ChoatieMom for most of the schools I’ve seen mentioned on this board, that’s true, which is why I was so shocked to see someone say that.
Thanks for the heads up on college tho!