Aslan says she will "find" him bigger, not that he will also grow bigger. It's all about perception. I don't think Aslan was invisible or smaller to them in a literal sense; just that they weren't perceiving him because of their doubts. They weren't looking for him, so they didn't see him. It's only when they follow Lucy down that path and realize they are headed the right way which enables them to perceive him. Simply because they have the evidence that Lucy was right and not imagining which clears their doubt and skepticism. They now realize Aslan's guidance and are able to perceive him.

Another example would be the dwarves in Last Battle, Aslan roared at them and they thought it some sort of machine that made the noise. He made a feast of wine and sumptuous food appear before them but they saw it as dirty water and grass. The dirty water and grass isn't what it was but that's how they CHOSE to perceive it. Also Uncle Andrew, every time Aslan spoke to him all he heard was roaring. Aslan said Andrew made himself unable to hear his voice. Our on willful perceptions can cast a stronger spell than any witch or wizard could. As Aslan said, if we won't be taken in then we can't be taken out.