| Quote #1
"I looked up and saw the very last thing I expected: a huge lion coming slowly towards me. And one queer thing was that there was no moon last night, but there was moonlight where the lion was. So it came nearer and nearer. I was terribly afraid of it. You may think that, being a dragon, I could have knocked any lion out easily enough. But it wasn't that kind of fear. I wasn't afraid of it eating me, I was just afraid of it – if you can understand." (7.34)
Eustace's fear of Aslan isn't a fear of any particular thing: Aslan simply inspires fear, in a good way. It's a deep awe that comes from recognizing something far more powerful than yourself and knowing that you have to submit to that power.
| Quote #2
Across the grey hillside above them – grey, for the heather was not yet in bloom – without noise, and without looking at them, and shining as if he were in bright sunlight though the sun had in fact gone in, passed with slow pace the hugest Lion that human eyes have ever seen. In describing the scene Lucy said afterwards, "He was the size of an elephant," though at another time she only said, "The size of a cart-horse." But it was not the size that mattered. Nobody dared to ask what it was. They knew it was Aslan. (8.82)
Lucy can't really explain how big Aslan was when he made this appearance, because his exact size doesn't really matter. He's just giving an impression of size and strength and seems bigger than normal. How much bigger isn't clear, and doesn't need to be.
| Quote #3
But when she looked back at the opening words of the spell, there in the middle of the writing, where she felt quite sure there had been no picture before, she found the great face of a lion, of the Lion, Aslan himself, staring into hers. It was painted such a bright gold that it seemed to be coming towards her out of the page; and indeed she never was quite sure afterwards that it hadn't really moved a little. At any rate she knew the expression on his face quite well. He was growling and you could see most of his teeth. She became horribly afraid and turned over the page at once. (10.22)
Lucy is afraid of Aslan at this moment, not because Aslan himself is terrible but because she knows she has been wishing for a terrible thing that would bring on Aslan's anger.