confidence intervals

Dear All,

Is there a rule of the thumb to judge if a 95% CI is wide or narrow?
If hypothetically I have a 95% CI for the difference in house price
means of £10 to £20, I know instinctively that this interval is
narrow, but if I have 95% CI for the difference in Cola price means of
£1 to £11, I know that this interval is wide. Is there a mathematical
method, rather than my opinion, to assess if these £10 intervals are
narrow or wide?

Thanks,
Thaís

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