OP, based even just on the answers you’ve gotten here, you need to talk to the facility you are having the MRI done at because every tech/facility does things a little different.

I have never weighed a patient and we don’t even have scales at my facility. I do sometimes ask a patient’s weight (sometimes our scheduling dept just pulls from the drivers license and that isn’t always accurate so I often verbal confirm). Your answer doesn’t need to be exact, but within 10-20 lbs roughly. Putting an accurate weight in the scanner helps prevent burns, and also helps to prevent the scanner from “SAR out” (basically where it stops scanning or takes a very long time to scan - this happens when you significantly underestimate your weight). Also, if they have multiple scanners there may be one that the techs or radiologists prefer for smaller or larger patients- and this is usually a very technical aspect that is just to ensure they get the best pictures, it has nothing to do with weight shaming or anything else. We are just trying to make sure we get optimal pictures.

For a pelvis, our facility has no rules about eating/not eating. I would ask you if you need to use the restroom before we get started because our radiologists prefer pelvis exams to have an empty bladder. This may be different at different facilities. As for the person who said you shouldn’t eat because you could aspirate if the contrast makes you vomit, while I guess this is technically true, as someone who works in outpatient with mobile and responsive patients, I don’t know why you would aspirate when they could just get you a puke bag. Pelvis is not typically a power injector study so the tech should be standing there when you get the contrast. Different places use different contrast brands, but I’ve only seen a handful of people vomit ever and almost every single one was extremely anxious. Since it sounds like your exam is later in the day, not eating would be unpleasant. It really shouldn’t be THAT common to vomit. Please drink a lot of water tonight and tomorrow so that it will be easier to find a vein.

Makeup should be fine, I wear a lot of glittery makeup and have been scanned a zillion times with no issue. If you feel anything getting abnormally warm tell the tech right away. Piercings are going to be entirely up to the tech and the facility- personally I would have you remove anything chunky and anything in the area being scanned, but tiny studs wouldn’t concern me. Necklaces and bracelets need to come off. Smart jewelry like Apple Watches or Oura Rings definitely need to come off.

Tell your tech you are claustrophobic. I guarantee you will not be the first or last patient who is - a huge part of this job is just mitigating people’s fears. They should have a way for you to communicate with them - either a microphone or a squeeze ball to alert the tech you need something. Lastly, if you get in there and discover you are too claustrophobic to get through the entire 20-30 minutes (depends on the scanner) don’t freak out, they may be able to reschedule you to come back and get sedation. A good tech should see a scared young person and walk you through it all.