1. At a big school, are you completely on your own?
NO!!! I don't know where or why this rumor started, but it's absolutely not true. I can't speak for other schools but I can speak for Purdue when I say that the professors want their students to succeed. In every experience of mine, professors and course coordinators have been happy to help me in any way necessary. While passing grades are not handed out, they are available to those willing to work hard.
2. Do you get all the classes you need? And how many classes are taught by TA's?
Yes. I get every class that I need. I might not get the times that I prefer (ugh, I hate getting stuck with 7:30's) but I get the classes I need. I was warned that classes could be cut-throat, but never in my experiences or in those of my friends. IF a course does fill up, many times new sections will open up. I really don't ever worry about that. As for the TA's, only the really popular gen-ed classes and recitations/labs are exclusively taught by TA's.
3. How bad are the dorms?
Not bad. Not great. Every building (and school, I'm sure) is different and there are pros and cons to everything. Yeah, the bathrooms aren't great. But the RAs are great and the social environment is like nothing you've known before. Some of my greatest memories of freshman year are from running around in the dorms, cramming 15 girls in a room for a movie night, and ordering cheesy bread at 1 am. They're only as bad as you make it.
I get so many questions, but these are among the most common. Again, these answers may not be universally correct. But I can't imagine these answers would be too far off. Be sure to ask any questions that you have- ambassadors volunteer to do admissions events for a reason! We love answering questions and making our experiences available.
Best of luck on your visits!