The size of the rollup interval depends on the time window at which you are currently looking. If you'd like to change the amount of seconds for a rollup interval, you can add the seconds parameter to the rollup function.
So if you are looking at the past 1 hour and wanted to see it rolled up into one minute intervals, you can use .rollup(avg, 60). Please note that for larger time intervals (such as a day), we have a default minimum rollup so we don’t stress our query processes.
More info on the rollup() function can be found here.