How do you say if someone is of "medium weight" or "medium height" ? Or is this correct ? Yes, it is but you can also say average (weight)(height).
And is there a rule for when to use "medium" and when to use "middle" ?
Terms like 'middleweight' are only used in boxing, as far as I know. Note that 'middleweight' is one word and is a noun.
Aside from the above, when to use 'medium' or 'middle' is a big question and sometimes both can be used.
'middle' is more commonly used, and is used for time (middle of the afternoon), space (in the middle of the room), and temperature, (it's been in the mid(dle) teens all week.
'medium' is used for cooking. (cook at medium heat for 30 minutes)(I like my steaks medium)
Check with a dictionary for other examples of their use.