They actually buy rings comparing with the some other rings or in jewelery shop they ll have fake rings for checking the size...
and they showed this kind scene in kuch toh log kayange serial...where ashutosh buys ring for nidhi ...and he wanted to know whether it fits her...and nidhi said it cannot be checked...so they use thread mark her size and compares with the actual ring...
creatives to increase the level of misunderstanding they showed that scene...so not to worry...