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Silca's FAQ section came in handy...
WHAT SIZE IMPERO ULTIMATE FRAME PUMP DO I NEED IF THE MEASUREMENT IS
RIGHT ON THE LINE BETWEEN TWO SIZES? If your measurement is right on
the line between sizes, we suggest that you go with the larger size.
For example, if your measurement is 50 cm, in between a medium and
large, we would suggest you get the size large. Keep in mind that the
minimum measurements are the most the pumps will compress so for this
example the large pump will not fix a frame with an inside measurement
of anything less than 50 cm.
They're great pumps, and the 'ask no questions' warranty makes great mental gymnastics maths possible ;) however they won't warranty it if you lose it so:
1.Big size is better for you as will be jammed in the frame
2.Matt can't have one as he loses everything