sizes (in everything ... from outerware down to socks and shoes)

Hi,

Just in case....
Could somebody please tell me if there are any ONLINE materials/atricles on this topic( = 'what language/nomenclature do English speakers use when talking about sizes they wear in clothes, headware, shoes)??

Too many unclear points here.....
For example, consider "I am a five and a half in shoes".
This grammar ( "to be a certain number in shoes") is not obvious, at least....

Thanks in advance!

mus-te