Writing Sample Help

[Crim2019]

All of the schools that I plan to apply to require a writing sample. I have a few questions:

1. I don't have a masters and I didn't do a thesis in undergrad - would it be okay if I just submitted a paper I did for a class?
2. My undergrad is in Psychology, would it be sufficient if I submit a paper with a forensic psychology focus?
3. Most of the schools don't specify length. The paper I'm thinking of submitting is 7 pages long. Is that long enough?

Thanks!