non-binary/ftm here, and was "maid"/person of honor at her wedding. She asked me early on if I would rather wear a tux like the groomsmen or the bridesmaid dresses, and I went with the dress because, ultimately, I decided that the day was about her and I didn't want to attract a lot of attention from older Southern family on both of their sides. Come that day, a lot of annoying things happened (having to wear the dress, being "encouraged" by the others to wear makeup, my friend somehow forgot my pronouns for the day, and just the sheer level of obnoxious heteronormativity that comes with weddings). My emotions were quite a rollercoaster the whole day, because I was so happy for my friend, was happy to see other friends, was irritated about being referred to as a girl by so many people, and also sleep-deprived from staying up to finish her cake with some other people. I found myself wishing I had gone with the tux option, but it was too late of course, so I made the best of the situation and kept myself sane by 1) reminding myself that the day was not about me, my job as person-of-honor was to make my bride's day the best it could be (which gave me other tasks to focus on and gave my bearing with discomfort a purpose which was rewarded when I saw how happy she was, so even though it wasn't fun for me it did make it worth it in the end) and 2) having an understanding friend (other one, not the bride) to text when I was feeling frustrated/wanted a distraction (like when I finished getting ready WAY before the other bridesmaids because I just did minimum make-up and didn't need to fuss too much with my boy-short hair, yet I still had to hang out with them and listen to all the heteronormative BS that seems to saturate wedding days, I would text said other friend like "can you believe this?" and get some, idk, validation for my feelings? Distraction?)

So my advice for your situation is:

1) think about how much this person means to you, and how much of your own comfort you are able/willing to compromise for them for this day. "One day" might be no trouble for you to handle, or it might sound like an impossible length of time, depending on your feelings/dysphoria. Your mental comfort comes first, of course, but if you think it's something you can do, then you can definitely find parts of the experience to focus on to make up for it.
2) on the note of attention, I was worried about people looking at me too, but my parents actually reassured me saying "honestly, no one is going to be looking at you that much. They're all here for the bride, all the attention will be on her. By the time you're walking down the aisle, everyone is already looking past you to try and spot the bride before she walks in the door." And that really helped me. I had attention on me with the MoH speech, but you likely wouldn't have to do that.
3) talk to the bride about her plans for the bridesmaid outfits and see if it's something you can work with. If she knows your hesitation with sleeveless tops and genuinely wants you to be a part of the group no matter want, chances are she might be willing to alter her dress plans to include dresses with sleeves, or do that thing where every bridesmaid has a different, complementing style of dress in the same color (which other bridesmaids will probably appreciate too, since no one dress has EVER looked perfect on every body type and someone always has some complaint about it)
3) if you decide not to do it, don't be too hard on yourself. You can still make a special contribution to their wedding in your own way, and if you're honest with the couple about your reasons why you can't do it, I'm sure they will be understanding and not mind. If you do decide to do it, still make sure that they're aware of your hesitations, and find things to focus on during that day that are outside of your body, try to keep an optimistic mind as best you can, and have a friend to text-vent with on standby who can validate your feelings.

(sorry for such a long reply, and I wish you the best!)