If you join GS1 you are covered. They are the standard and any retailer you do business with will be able to read your bar code. Also by getting your bar codes from them, you are guaranteed to have unique bar codes. There are companies out there selling single bar codes that have already been issued, and that causes major problems.
Yes, size does matter. You will need to determine the size. You can check out the GS1 website for sizes. Normally the size is more proportional that anything. Meaning that as long as the height is always in the same proportion to the width, then the actual size wouldn't matter much. You could make it as big or as small as you needed.
Also you can contact them directly with questions. They are pretty good about helping with that kind of stuff.
Here's the link to the sizing info:
10 Steps to Bar Code Implementation
Hope this helps.