The standard cabin bag size is generally referred to as the "carry-on" luggage size. The size requirements may vary depending on the airline, as each airline may have their own restrictions. However, typically, most domestic airlines will allow a bag no larger than 45 inches (length + width + height).
Here are some other general guidelines to keep in mind when selecting a cabin bag size:
The bag must fit in the overhead bin: Airlines may have different luggage bin sizing, so make sure to check the size of your bag against the airline restrictions.
Weight limitations: The maximum combined weight of your carry on and personal item (purse, laptop bag, etc) is typically 40lbs. Anything heavier may incur an additional fee.
Consider your destination: Some countries may have their own limitations on the size of carry on items. Make sure to check with the airline ahead of time for any size restrictions.
Check with your airline: The best place to check luggage size restrictions is directly with the airline. You can typically find the information on their website or with a customer service agent.
For more information, here is a helpful link to the specific size and allowance guidelines for many domestic airlines: https://www.airlines-inform.com/size-chart.html#.