Generally speaking, most airlines have a max carry on size of 22 inches tall, 14 inches wide, and 9 inches deep, but don't quote me on that, because like I said, each airline is like a snowflake - unique and annoyingly different. Some airlines are super strict, while others are more laid back, so it's always best to check with your airline before you start packing. And trust me, you don't want to be that guy who gets to the gate and has to gate-check their baby (aka their carry on bag).
Now, you might be wondering, why do airlines even have max carry on size restrictions in the first place? Well, it's not just because they're meanies who want to ruin your travel vibes. It's actually for safety reasons, and to make sure everyone's bags fit in the overhead bins without causing a Carry On-geddon. I mean, can you imagine if everyone had a giant bag? It would be like a Clash of the Titans up in there!
But here's the thing, some airlines are more generous than others when it comes to max carry on size. For example, Delta and American Airlines have a pretty standard size limit, while Southwest is a bit more relaxed (because, you know, they're Southwest - they're all about being chill). And then there are the fancy airlines like Emirates and Qatar, which have their own set of rules and regulations (and probably their own carry on butlers to help you with your bags).