Which is the smallest sounding rocket to reach space (100 km)? By height
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The definition of "reaching space", in contrast to orbit is a bit arbitrary. The Kármán line of 100km is often used.
An amateur rocket launch, but the USC rocket team reached 103.6km with their Traveler IV. They list their rocket as 13 foot tall, or around 4 metres.
That gives an upper bound.
If you consider 72km sufficient, the Black Brant VI and VII rockets can reach that altitude with a 2.80m launch vehicle.