Short answer: There is no single standard. Many airlines allow a bag of about 55 × 35 × 20 to 25 cm and 7 kg in economy, but limits vary, so check your ticket and measure the bag with its wheels and handles.
There is no single worldwide rule for cabin bags. Each airline sets its own, but most stay close to a common standard.
Size
The airline industry body IATA suggests a maximum of 56 × 45 × 25 cm, but that is an upper guideline, not a promise. Many airlines in Asia and the Middle East allow less, often around 55 × 35 × 20 to 25 cm, and some low-cost fares include only a small under-seat bag. The limit always includes wheels, handles and outside pockets, so measure the whole bag against your airline's figure before you buy one.
Weight
Weight limits vary much more than size. In economy, 7 kg is common across Asia, while some airlines allow 8 or 10 kg. Business class usually allows more, sometimes two bags.
The personal item
Most airlines also allow a small personal item, such as a handbag, laptop bag or small backpack, that fits under the seat in front. Check whether it counts towards your weight limit: on many airlines it does.
Tip: Weigh your packed bag at home. A cheap luggage scale costs far less than one excess baggage charge at the gate.
What has to go in hand luggage
Some things must travel with you, not in the hold: power banks and spare lithium batteries, plus anything valuable or essential such as medicines, documents and keys. Liquids in hand luggage follow the 100 ml rule.
Connecting flights
On a journey with two airlines, the stricter allowance often applies to the whole trip. Check both, especially if one is a low-cost carrier.
Questions people ask
Q: Do wheels count in the size limit?
A: Yes. Airline limits include wheels, handles and outside pockets, so measure the whole bag.
Q: What happens if my bag is too heavy at the gate?
A: You may have to pay to check it in, or move items into your personal item or pockets. Gate fees are usually higher than paying for baggage in advance.