DXM doses are WEIGHT DEPENDENT. You need x mg of DXM per y kg of body weight to achieve a certain plateau, assuming you are not enzyme deficient. The charts for this are in the DXM FAQ, I think section 5.1-- sec... yeah, DXM FAQ Section 5.1. The first and second lines of the table are the most important as they are the "general range" for each plateau, as well as a recommended dosage. So let's say you weigh 245 pounds. Go to google.com. Type in "convert 245 pounds to kg". The results page will say: "245 pounds = 111.130131 kilograms". Call it 110. Now look at the chart. Say you want a light first plateau experience, just a bit of DXM intoxication, but no dissociation or hallucinations. 1.5mg/kg * 110kg = 165mg of DXM. Say you want a deep third plateau experience with lots of blacking out and crazy hallucinations (this is more fun than it sounds). 12mg/kg * 110kg = 1,320mg of DXM. As you can see, that's a lot of DXM to try to take in pills, and it's a dangerous amount if you're using a syrup or something that contains another active ingredient. Some of these can kill you, like acetaminophen or chlorpheniramine malleate. Others like guafenesin will just make you vomit uncontrollably at that level. That's when you want to read about extractions (google "Agent Lemon Tek") or finding preparations that ONLY contain DXM.
Also, I attach my "stock DXM response":
A large percentage of the population have an enzyme deficiency that will make drugs like DXM hit them harder. These are the people who mention how they took a little cough syrup for a cold, and it completely spaced them out. For these people the effects of DXM are greatly magnified, and what we might consider a low recreational dose (300-400mg) could be life threatening, or at least incredibly unpleasant. "In one study on 252 Americans, 84.3% were found to be "fast" (extensive) metabolizers, 6.8% to be "intermediate" metabolizers, and 8.8% were "slow" metabolizers of DXM." Take 100mg a couple of days before you want to trip in earnest to make sure you're not one of the 1 in 10.
Note that SSRI antidepressants can also inhibit this enzyme, so if you are on one (Prozac, Lexapro, Celexa, Paxil, Zoloft, etc, etc.) you should reconsider taking DXM. Finally, DXM and opiates use the same metabolic pathway and can synergize. Mixing DXM with pain medication is not recommended without a thorough understanding of how each effects you by itself.
Ignore anyone who tells you how much DXM to take without asking what level of experience you want, and how much you weigh.