The setting for choosing maximum and average rates is right there, in the Speed Meter window, as highlighted in the screen shot below.
Regarding the monitoring period and number of tries: the test starts with 1 MB buffer and a download/upload is performed. Then the buffer is doubled to 2 MB, 4 MB, 16 MB and so on until the operation takes 10 or more seconds. Once we have a large enough sample for the download/upload to last for at least 10 seconds, we calculate the average and maximum values during that period.