Hi! There is no direct way to request length, but you can get it pretty easily. Follow this documentation for example requests. You can make a function that subtracts the start time of the meeting from the end time.
"start": {
"dateTime": "2017-08-14T21:00:00.0000000",
"timeZone": "UTC"
},
"end": {
"dateTime": "2017-08-14T22:00:00.0000000",
"timeZone": "UTC"
},
I hope this helps! Please let me know if you have any other questions.