MML trivia I need clarified...

Denise Marmalade, how old is she? She's a cop, but she looks very young... Is she 20ish? thus making her around 22 for MML3?

How tall are the characters? I can't seem to find that info anywhere...

how long ago did Trigger and Sera have their big fight, and who sealed them in the big octahedrons?

I've never found a site that says how old Denise is, but I'm guessing eighteen - twenty 'ish. Tron and Volnutt are both between fourteen and fifteen, so it'd seem like Denise should be young also. But in order to be a Cop, you've got to be eighteen, so she's got to older than seventeen... Then again, not everything made a lot of sense in Megaman Legends, age wise.
Never have I found an answer to the second question, either. But I'll infer like I did my first answer; Megaman is fourteen 'ish, and I was about 5'11" when I was fourteen (But I was on the taller side). Megaman seems like he's a little on the shorter side, so maybe 5'7"? You could assume that all the adult male characters are the average height (5'10") and the adult women character are the average female height (5'4").
If no one else answers your last question, I'll get back to you once I finish Megaman Legends 2.
It never gives an exact date, but according to "The MegaMan Knowledge Database" it happens after the Master dies on Terra, Trigger tried to shut down the Master System, and Data was created; And takes place before Terra is flooded.

(Most of) My answers are based on estimates, but I hope I helped answer the questions a little bit.

I remember there were mentioned, "thousands of years" in reference to most of the things related to the Master... He lived 3000 years, but how long ago did he die... Essentially, how long was megaman in stasis, was it a few years? Decades? Centuries?

I know it coincides with Yuna living on the forbidden island to deter travelers, but again, I don't recall it mentioned how long she was doing that before she met Matilda...