Auto-Resize

I'm not sure if this is building or scripting, so I'll put it here and hope for the best.
I've been working on a belt-type accessory (with the necessary dangly and shiny bits) but each time I change it even slightly, I have to move it exactly to the right place if I want to wear it, and if I change avatar size, it screws up completely. Is there a way to have it automatically resize itself to the waist size etc of my avatar when I wear it?

I'm not aware of any method of automatically determining an avatar's waist size, but if it exists, I'd love to know it. In the mean time, you can adjust that belt size without having to reposition it. Right click on it while you're wearing it, and select Edit from the pie menu. On the editor, make sure "Stretch both sides" is turned on. Drag any one of the white corner manipulators (not the colored side manipulators), and it should scale uniformally, while remaining in place.

If you change your avatar height, you still may need to move the belt a bit. I can't get inworld to test this right now since the grid is currently closed, but I'm 99% sure that offset values for attachments are measured in absolute distances, not relative distances. If your belt is 10 cm away from the attachment point, it might look right on on avatar that's 2 meters tall, but shrink that avatar to one meter tall, and the belt will appear to be in the wrong place since its distance from the attachment point won't be changed. You'll have to manually move the belt into the proper position.

Just so you know, you may want to invest in a posing stand to make editing attachments easier. It will stop all avatar movement when you stand on it, so you can edit attachments precisely, without having to worry about the avatar moving around.