Sometimes the smallest size is medium. I have never yet thought to ask what it is the medium of at a time when doing so wouldn't make me feel guilty for delaying the people behind me.

Variations on a theme:

'We have extra small, small and regular.' Espresso shots, perhaps?

'We have extra small, extra regular and extra large.'

'We have fine, diminutive, tiny, small, medium, large, huge, gargantuan and colossal.' Geek humour. Sorry.

'We only have large.'

'We have half-pints, American pints and British pints.'

'We're obliged by European law to use certain measures. You can have a lille, a moyen, a grand or an extra grande. Guten tag!'