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Little enough too big that once I'd (noticed and) filed off the little bur from the front of the seat cluster lug a 27.2 would fit in so, problem solved.
I've a cheap set of digitals at home which currently have some sort of issue with the digital display going mental but at least they can be read manually however I'm currently at work where we have the Park ones which have a flat battery and are not so easy to read manually as there's nothing actually near the manual scale to take a reading from.
How much is 27.2 too big? I've had steel frames at 27.0 but very little that wasn't 27.2.
Invest in some good vernier calipers. Don't go digital unless you're spending serious money though. But don't get the cheapest manual ones either.
In a pinch a ruler should give you a better idea.