52-16 is high gearing. most people here seem to ride around 48-16 to 48-18 for city riding. not sure why the guy in the shop reckoned you couldn't put a smaller cog on your freewheel, though. maybe try in another shop - the first guy may have just run out of smaller cogs. someone else may know of a possible reason.
i would start with a 16 or 17.
52-16 is high gearing. most people here seem to ride around 48-16 to 48-18 for city riding. not sure why the guy in the shop reckoned you couldn't put a smaller cog on your freewheel, though. maybe try in another shop - the first guy may have just run out of smaller cogs. someone else may know of a possible reason.