4.25, could potentially size down to a 4. Nothing comes off, but my rings slide around a lot. I have a guard on my 4.5 engagement ring and they didn't want to size my band lower than a 4.25
My rings are 10.5 but they’re tight. I can get my e-ring on and off - most days - but the weddings bands I almost had to get cut off when I tried them on once.