1. Which punctuation mark should we use when we have two independent clauses that have (show) a relationship between them? Would a colon be correct?--
No. A semicolon or a full stop. (I presume that you are speaking of two clauses with no coordinating conjunction.)e.g.
He has cookies. I don't have any.
He has cookies; I don't have any.
2. Do you think what is in parentheses is implicit in the structure and the sentence is correct (in addition to the verb "was" in the underlined part?-- Implicit through common sense, but in this sort of situation, you should write it out to assure clarity– at least the indication 'total':
The total length was XXX and the total width XXX.