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There are no standards for baseboard systems in general because each company's system has a somewhat different heat output per foot of baseboard. Therefore, this information is best obtained direct from the manufacturer. They should be able to supply a formula to determine how much baseboard is optimal for your rooms.
Optimal, however, does not mean you will have a tight temperature range in all the rooms. When rooms are all different sizes as you describe, I think it would be wise to put each room on a separate zone, especially if temperature consistency is very important to you.
This is a complicated issue. Let's say your two smaller bedrooms have different length baseboards. If the first room in the zone "loop" has a longer baseboard, by the time the heated water reached the second room it would no longer be as hot! But just how hot?? We can hypothesize on various scenarios but you can see how it really is a guessing game!
Remember that baseboard heating was developed based on rooms with similar ceiling heights... the most critical factor. Homes with high, vaulted or cathedral ceilings are consistently inconsistent in the temperature department (summer or winter) unless there are multiple zones. It's just a fact of life!