Is the Loop really big, or can I walk around it fairly easily? Will I need a cab?

You can walk around the Loop, but it can take a while to get from one end to the other. On one of my first dates with my girlfriend we went to Bennigan's on Michigan Avenue, then walked over to Navy Pier to ride the Ferris Wheel. We kinda got lost, which is ridiculous since we could see the Ferris Wheel half the time, but it took us a good hour to get there. It was about 40 minutes back. So you can do it, but it takes time.