The short answer is “you can’t”. There is no way that you can ever tell anyone that they are “obnoxious” (or even close to that, or with euphemisms) without offending them.
And for you to attempt to tell her that she’s “inappropriate” is to apply your standards to her, which she already doesn’t accept, or she wouldn’t do the things you notice if she knew that you were noticing. For example, most of us think that it’s inappropriate to “masturbate loudly and violently” and if we knew that our roommates could hear us, would at least attempt to modify our behavior. The fact that she apparently knows that you know, and doesn’t care, means that “your standards” don’t apply to her. (That doesn’t mean that you’re racist, but that’s often a convenient dodge when one doesn’t want to change behavior: “you’re wrong for wanting me to change”.)
What you might actually be able to do is negotiate with her. If you tell her one thing that you’d like her to change – just because you want her to – and agree to change something about yourself at the same time – then you might start to make some progress. But you really need to be diplomatic about this, and maybe even offer to make the first change in yourself, to satisfy some desire of hers.
If the relationship is already as toxic as it seems, then even that may be a long shot, but the only one likely to work if you’re going to have to continue to live together.
I must add, though, that you’re bringing things into this description that indicate a certain unwillingness on your part to accept things as they are, such as her weight, for example. Are you absolutely sure that some of your antipathy toward her doesn’t stem from things that have less to do with “behavior” and more to do with some kind of “appearance”?