In the US, asking about someone's weight or age is usually considered impolite (especially if you are asking a woman). Asking about height is ususally not offensive.

I think that the idea is not that "it is alright to ask about ur height but impolite about weight" The idea is that if a person (especially a woman) is too short or tall or large, asking about her qualities that (s)he thinks to be imperfect is really impolite. But if her height or weight is perfect - asking such questions are often like complimenting.

asking about height is ok but weight is more personal to people and they may be offended

People don't ask these questions during conversation, it is not considered polite in western countries.

Usually asking for someone's weight is rude, but people don't really care if you ask them what their height is.