I personally took 360mg. I'm pretty slim though (125lbs, 5'10"). It was a pretty intense experience.
That said, this was my first ever trip. I would not recommend just taking my advice. Maybe create a topic here about it? I'm sure there are people willing to help you out :)
Have fun and be careful! DXM proved to be incredibly intense and is not something to mess around with :p
I DO however recommend this as a source for DXM. This is 30mg of DXM per pill. I took 360mg so that's 12 pills. I was told for my size, an intense trip would be 360mg, so I decided to do that (it was worth it). My friend who is larger takes 480mg for an intense trip. Seems like an intense trip would be around that range.