Just FYI, I've only bought one size for >10 years.
Well, two sizes if you count mountain bikes. But we have road tires in the family ranging from 700x23 to 41 and MB tires from 26x 1.25 to 2.3. I use 700x25 or 26x1.5 tubes in everything, and I've never had a problem.
It's better to be a little small than too big, IMO. A way-too-big tube will occasionally wrinkle and can wear a hole in itself. But close enough is close enough.
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