Ok kiddo, do you know how sometimes we want to play with a toy that's too big or heavy for us to pick up? That's kind of like limitations of size.
Sometimes things can only be a certain size because of how they were made or the materials they're made of. Like imagine trying to make a giant teddy bear out of only a little bit of fabric - it wouldn't be possible because there's not enough fabric to make it big.
In real life, there are also limitations of size for things like buildings, vehicles, and even living creatures. These limitations depend on how much space there is, how strong the materials are, and what kind of environment they're in.
So a building can only be so tall, because if it's too tall, it might fall over or the materials might not be strong enough to hold it up. And a car can only be so big because it needs to fit on the road and not be too heavy for the engine to move it. Even animals have limitations of size - they can only grow as big as their species allows, because their bodies can't support being any bigger than that.
So basically, "limitations of size" means that sometimes things can only be a certain size because of physical or practical limitations.