Here are some resources for you -
https://www.scirra.com/tutorials/73/sup ... reen-sizes
https://www.scirra.com/blog/112/remembe ... our-memory
This isn't a simple answer about what is best - you really have to work in your design limitations and goals.
700x700 will take up that much image memory regardless of resizing. Maybe you don't have that many walls and memory is not an issue for you. Are you downsizing all your walls in the game anyway? If so then the extra resolution is wasted (Might even look worse!).
One suggestion that almost always helps is to keep the size your images in power of 2s.
Generally speaking, use smaller images and upsize in the engine, but there is a quality tradeoff to consider.
Otherwise, it would be ideal to have the image size exactly the size you need to display based on the resolution you are designing for.