I read a book back in the 80s, Principles for a Catholic Morality, Timothy E O’Connell that I thought was very good. He goes through all the various ways of classifying sins, points out their strengths and weaknesses, etc. I think it gives a very good overview of the subject.
The catechism also has things to say, and since it’s online, that’s easy to check.
I would also consult my own head, much as several other commenters said. Using their example, let’s say you steal a candy bar worth 85 cents from a store. There are a host of ways to look at this: you are depriving the owner of a living, you might be starving, you steal a candy bar every day from this store, you are a kleptomaniac and can’t help yourself from stealing at least one item from every store you go in, you are giving a bad example to some kid who saw you steal it, etc. etc.
So you can easily see it quickly becomes very, very complicated. But there are a couple reasonable comments you can make: First, the Church–or police–aren’t going to say, “Stealing is OK.” A confessor or judge might find out some other circumstances and treat each individual differently. But ultimately, it’s God–who knows ALL the details–who is the judge. But the Church is in the business of publishing general rules for the believers at large, and that makes perfect sense, just as the government makes laws for everyone, realizing that judges have the power to overrule in certain cases.
The second thing is something I would not ordinarily even think of, but something I have seen an awful lot of on this site–scrupulosity. You’ve got to keep things in proportion. You can’t drive yourself nuts over every little thing that pops up–this is not how God wants you to live your life. Each of us has some weakness or weak area. I drink, but seldom–I often go months without a drink, and it doesn’t bother me at all. And when I do drink, it’s only a couple beers or a glass of wine, or maybe one cocktail. So for me that’s not an area of concern. But let’s say I have an issue with stealing–office supplies from work, tools from worksites, things from a store when the clerk is in the back, etc. In that case, I need to understand that I DO have a problem, and in that case (going back to the candy bar above), even stealing an 85 cent candy bar is a big deal–FOR ME.
So is a particular sin “petty”? Maybe. It depends on a host of circumstances. But if it is part of a larger pattern, it’s probably noteworthy even if it is petty in itself. You need to know yourself–which is the object of an examination of conscience. Personally, I would pay more attention to the commandment to love your neighbor as yourself. Are you polite? Helpful? Honest in your dealings with others? Do you show appreciation for the help of others? Do you foster a workplace that is cooperative and honest? From my own life, I see these as the areas that matter, but are overlooked. Remember the Confiteor: “I have sinned…in what I HAVE FAILED TO DO.” It’s all very well and good to not steal that 85 cent candy bar, but if you then turn up at your job in Wall Street and sit there like a lump while the VP outlines his grand plan to buy derivatives and defraud millions of people of their savings, you’ve got a much bigger problem.
