How are 사업 and 장사 different?

It’s about the size or scope of the business. A small 옷 가게 (clothing store) at the local street market? Probably 장사. The mighty Lotte department store? Definitely 사업. But you can’t go wrong asking the owner of a business of any size “요즘 사업 잘 됩니까?” (How’s business these days?)