I'm a size 4 1/2 but always wear a size 3 in Birkenstocks narrow footbed; my feet are narrow but I have a v high instep (no I wasn't a cruise-ship dancer :tongue
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I have the EVA Birkis croc look-alikes and they're v comfortable and don't cause blisters or sweating (and wash well in the washing machine at 30degrees) so I've ordered the TSV in purple size 3. On the models there is a huge amount of room above the tops of their feet even on the smallest buckle setting so I'm confident they'll fit. If not they'll be cheap to return under the DSR.
They make good slippers.
Jude xx