>>1) Is there any size difference between males and females?
Males tend to be slightly larger. That being said, I have a female who is just about 10 inches long, and the species as a whole is supposed to average 8-10 inches, so your mileage may vary on that one.
>>2) Is one gender more aggressive than the other?
Supposedly males are slightly calmer, but I've been bitten exactly one time by my female, and she did that when she was a juvenile and hadn't completely learned to trust me yet. In general, once leos get comfortable with you, they're very docile.
>>3) Purchasing my first leo, does it matter which gender it is? Probably not, but I was just wondering if there was little kinks about attaining a male or attaining a female.
The only "kink" that I'm aware of is that you absolutely cannot house 2 males together, as they will fight and severely injure and possibly even kill each other.
>>4) How big of an enclosure for one adult leo?
10 gallons minimum, mine lives in a 20 gallon long.
>>5) how big of an enclosure for 2 adult m/f leo's?
20-30 gallons would be appropriate, bigger is of course ok too if you don't mind cleaning it.
>>6) Do the patterns and intensity of colors stay on the leo throughout its life time? Or does it fade with age?
Once they get their adult coloration and pattern, it should stick around through their whole lifetime. Of course they fade when they're going into shed, but that only lasts a couple of days at a time.
>>7) How nocturnal are they? Are we talking, "sleep 10-20 hours a day" nocturnal? Will I be seeing more activity from them once the sun goes down or when?
This can vary somewhat gecko to gecko. Mine tends to be fairly active at night, but probably does sleep (or at least, hide) solidly 12 hours/day. You will not see them with any regularity during the day, but if you've got at least most of the lights turned out in the room they're in, they'll be out exploring a lot of the time at night. Occasionally they'll come out during the day to get a drink of water, but most of the time they just hide during the day. Sometimes they will do weird things like in my photo, but that's the exception, not the rule. She hasn't done this in over a year if I recall correctly (photo taken middle of the day):

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0.1 Albino Leopard Gecko - Tigger
0.1 Crested Gecko - Pooh-Bear