We
will be joining the Rick Steves' Sicily Tour in late October. Is it acceptable to wear shorts during the day and/or to dinner?
Wear what you want. Nobody cares. Really, nobody in the word cares what you wear. At. All.
The only exceptions would be in some churches or other religious sites - where they will typically have a little sign reminding visitors to cover up a little (no shorts, mini-skirts, bare shoulders. etc.). And perhaps if planning to go to a fancy, high-end restaurant for dinner. If you're on a RS tour, ask the guide about the night's dinner venue. I suspect there will be none you encounter that wouldn't be OK with shorts.
Note that these "dress codes" are the same as you would experience at home - trust your common sense, it works the same there.
For the most part, Sicily is not too fancy. The best local restaurants we ate at were local and low key. However, the locals seemed very proud and modest.
Sicily is awesome … we joined a RS Sicily tour group last October. The weather can vary. My wife did a photo blog of our tour … you can view at graciamc.wordpress.com. If nothing else, you'll see the kind of clothes the group is wearing. Our tour was Oct 19 - Nov 1.