First of all, make sure that you're sure about wanting to be in a video. Most videos involve fairly strict bondage and intense tickling. You wouldn't want to stumble into a situation that you wouldn't want 2 minutes in.
Second, decide what kind of video you want to be in. That would likely determine which producers you'd want to contact.
Third, consider your location. Are you willing to pay for your own travel? Do you live near a major city, which is more likely to have producers?
Fourth, start making contacts. Many producers are members of the forum. Send your info out, let them know you're interested. I think most producers would jump at the chance to work with a TMF member who wants to explore their fetish on camera.
Fifth, and finally, don't worry about weight. If a producer is going to decide not to work with you based on your size, then that's likely a producer you don't want to work with in the first place.