They do climb in captivity if given the opportunity but they do not need to climb. It may be beneficial exercise. It is not something I've seen kings do often though.
If you are going to use overhead heat, taller means a higher wattage heating element. If you are going to use under tank heat, it does not matter that much.
A good "mid sized" king that is extremely beautiful would be the mountain kings, such as a California Mountain King (in CA only the Baja subspecies is legal to buy, with exception of L z pulchra you can keep the others but you have to collect them or be gifted - and no more than one w/o permit, pulchra can only be kept with permit) or Arizona Mountain King.
I'm guessing they are less likely to climb that common kings, but I don't know that for fact.
With Common Kings, Eastern / Florida kings seem to get the biggest, with six foot specimens not uncommon.
Milk snakes (which are king snakes) - North American varieties are small or mid sized, some of the tropical Central American varieties regularly hit six feet in captivity.
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Arrrggg!
It's like Shalom, but for pirates.
- iCarly