If a creature is a bigger size should I get a higher accuracy, or advantage?
A creatures size is directly correlated into their AC, along with armor, stats, and abilities. Whatever their listed AC is, that's what you have to beat, simply being larger does not grant you advantage on attacks against them
Under normal circumstances, there are no benefits to fighting a larger creature. However, there is an optional maneuver in the DMG that might be what you are desiring if your DM approves:
As an alternative, a suitably large opponent can be treated as terrain for the purpose of jumping onto its back or clinging to a limb. After making any ability checks necessary to get into position and onto the larger creature, the smaller creature uses its action to make a Strength (Athletics) or Dexterity (Acrobatics) check contested by the target's Dexterity (Acrobatics) check. If it wins the contest, the smaller creature successfully moves into the target creature's space and clings to its body. While in the target's space, the smaller creature moves with the target and has advantage on attack rolls against it. Page 271 of the DMG.
For Medium creatures, this might be available against Huge and Gargantuan creatures. For Small creatures (gnomes, halflings), this extends to Large.
AC, like HP and many other elements of the game, is an abstraction. It does not purely denote an opponent's ability to avoid getting it. It is a simplified way to determine if an attack as successful at bringing one closer to death. It might be from missing due to the opponent evading, or due to a magic displacement effect. It could be that the attack connects, but is too weak to pierce the opponent's armor or skin (this bumps up against DR, but an abstraction can having multiple levels). It could be a little of both, the opponent moves far enough out of the way that the attack bounces lightly off the opponent's shield. If the opponent hadn't had the shield, the attack would have cut their arm (armor bonus to AC), or if they hadn't partially evaded, the attack would have hit the shield harder, numbing the opponent's arm and ringing in her ears.
So just because the dragon is the size of a small apartment complex, and you are swinging your sword from two feet away, doesn't mean you have the skill and luck to cut between its scales instead of just hitting the scale dead on and doing nothing.
There is a good example in the PHB p. 213. Look for the Animate Objects spell. There is a table where you can see that the size affects the Dexterity which in turn affects the AC. It also shows that the bigger the stronger a creature gets.
Obviously, this table is not set in stone, but I still think it gives a general idea of what happens in correlation with the size of a creature.
However, there is no mention of Advantage or Disadvantage in any way just because of size. Even a creature that does not have a displacement, such as a Water Weird (MM p. 299) in a small fountain does not give advantage.
As the other answers have mentioned, there is no official rule.
However, I play with a house rule where you have +5 on attack rolls against creatures that are Gargantuan (controlling a 20-feet-by-20-feet space or larger). However, if you match the creature's AC with the +5 roll (in other words, it is because of this house rule that you were able to hit), then the creature takes only half damage from the attack. I just think it makes no sense to be about five feet away from a creature that is about 20 feet wide and just miss; however, I can understand its defense being too strong, but that usually isn't the case when I describe attacks.