Subfield is extremely important in determining the answers to these questions.
The size of the program is not a big deal. No matter what subfield, there are a ton of faculty members who have interests in tons of different areas. Some of your classes are going to be bigger than at other programs, which has its ups and downs. When you have bigger classes, it is really easy to find study groups and support networks throughout your cohort-- which is something that I found to be really important in helping me get through my first year. When you have smaller classes, there are different expectations to participation-- but it is completely dependent upon the expectations of the professors. Also, a lot of it will come down to your cohort. There are cohorts here that are less than 9 people and some that are more than 20.
And, as long as you don't hide away from professors and you seek them out, they are willing to mentor and give input on whatever you need. A lot of it is just being proactive.
I, personally, interact with 5-6 professors on a daily/weekly basis and was invited to work on personal research my first quarter in.
Living off the stipend is just fine. The graduate school named the PS program one of the best graduate programs on campus and upped the amount of the stipends for everyone in my cohort. I never feel like I'm sacrificing everything fun and unnecessary to survive on the stipend. I get enough where I can save money month to month.
Grads RA/TA choices are dependent on subfield. If you have a strong desire to do one or the other-- you will most likely get to choose what you want to do. Comparativists will most likely have to TA more than Americanists for example; however, it depends on the size of your cohort and the needs of the department. No matter what, you will get to do both eventually.
Lastly, you will get to teach classes; for most, it's a requirement.