Hun i think length of belt varies depending on airline...off the top of my head i think ours are 42inches, but it sits around widest part so not waist size but i'll find out for def for you if you like!
I know nothing will prob make you feel better about the situation should you need one but i really can't emphasise enough how unimortant it is to crew, like i said in other thread i have never ever heard any laughing/jokes/mocking or any such thing about someone needing one. It is so common these days that i guarentee you if you do need one you will not be the only one on the plane, and crew are so so busy during boarding stage that they won't bat an eyelid. It may be mortifying to the person needing the extension but on the whole people don't care about what is going on unless it affects them.
To try and ease your mind i once flew with a girl who was a complete cow, just making nasty remarks about people the whole trip..hair colour, 'ugly men', even commenting on the 'whale' (her words not mine on the beach). She really wound a lot of us crew up the whole trip with her persistant bitching...on the way home she had an Americian family who were all big, when i say big i mean huge..the father had to buy an upper class ticket as he would never of fitted into an economy one....anyway she gave them the belts then came into the back galley and me and another girl were just waiting for her to ***** as we were gonna give her a mouthful about being proffessional........i kid ya not, nobody was more shocked than me that she didn't utter a word, i don't think she was being nice but i just think she hadn't registered.
Hope you understand what i'm getting at with that story, and that it may make you feel a bit better IF you need one, wish you were on my flight, i'd look after you!!xx