How can i known when is a plural?...
"Kemeja mereka besar."
It's based on the word 'mereka' ('they').
More than one person ==> more than one shirt.
'Their shirt is big' (singular shirt) is also on the list of accepted translations.
I'm not sure if that makes sense, or if it's grammatically correct English.
It sounds strange to me, more than one person and just one shirt...