"There'd be no scope for imagination then." Here "scope" means

“When we got into the train at Charlottetown and the red roads began to flash past I asked Mrs. Spencer what made them red and she said she didn’t know and for pity’s sake not to ask her any more questions. She said I must have asked her a thousand already. I suppose I had, too, but how are you going to find out about things if you don’t ask questions? And what does make the roads red?”

“Well now, I dunno,” said Matthew.

“Well, that is one of the things to find out sometime. Isn’t it splendid to think of all the things there are to find out about? It just makes me feel glad to be alive—it’s such an interesting world. It wouldn’t be half so interesting if we knew all about everything, would it? There’d be no scope for imagination then, would there? But am I talking too much? Would you rather I didn’t talk? If you say so I’ll stop. I can stop when I make up my mind to it, although it’s difficult.”

Matthew, much to his own surprise, was enjoying himself. But he had never expected to enjoy the society of a little girl. He detested the way they had of sidling past him timidly, with sidewise glances, as if they expected him to gobble them up at a mouthful if they ventured to say a word. That was the Avonlea type of well-bred little girl. But this freckled witch was very different, and although he found it rather difficult for his slower intelligence to keep up with her brisk mental processes he thought that he “kind of liked her chatter.” So he said as shyly as usual:

“Oh, you can talk as much as you like. I don’t mind.”