1. TRIPLE check the size constraints for every single airline you'll be flying on your trip so you aren't forced to gate-check an oversized (according to the airline) bag. Budget airlines have stricter constraints on luggage size (looking at you, Spirit Airlines and EasyJet ) because, well, you get what you pay for and they've gotta get you $$$-wise somewhere if not for the face price of the plane ticket.
BUT, you can sometimes pay to carry-on a bag that doesn't fit their FREE.99 baggage allowances or are allowed one depending on the type of ticket you book . For example, a few years ago I paid for priority boarding and to take on my roller carry-on on an EasyJet flight from Palma de Mallorca to Gatwick, and then from Gatwick to Rhodes, Greece.
It was worth the peace of mind to have my stuff with me and not risk the airline losing my bag (which has been happening QUITE frequently lately, in case you've been living in a cave). Plus! Make sure that you're measuring correctly when planning ahead. Confusing centimeters for inches would be downright tragic.
2. Just because your bag fit LAST time doesn't mean that it fits this time! If you ever cross into TravelTok, you've probably already seen videos about sizers changing in dimensions at certain airports with vigilant commenters! (Yep, I am one of those vigilant commenters.) 3.
Also use a digital luggage scale to make sure that you aren't slammed with an overweight luggage fee (ugh) or having to move stuff from your carryon bag t.