I considered that, but didn't want to have users see that the default is way bigger / smaller than their measurement, especially since my target audience people who can't find patterns that fit their particular sizes.. any suggestions?

I could put in better error handling to avoid measurements that are physically impossible

That's true, maybe a range input would work though (slider control) it would be nice from a usability point of view.

If people can quickly go through the steps and see a preview, I don't think they'll be Ashamed of the default sizes, but I'm not the target audience I guess...

Yeah, getting people to the last step more quickly would be ideal, maybe a slider is the way to go. thanks!