Understanding sampling rate and frequency of logic analysers
The leftmost setting it the total number of samples per capture while the rightmost option selects the sampling rate.

1M samples, @16MHz : will gather 1M samples total with a rate of 16M samples per second. That means that the capture duration will be (1/16)=0.0625 second (1M/16M)

16M samples, @16MHz : will gather 16M samples total with a rate of 16M samples per second. That means that the capture duration will be (16/16)=1 seconds (16M/16M)

32M samples, @16MHz : will gather 32M samples total with a rate of 16M samples per second. That means that the capture duration will be (32/16)=2 seconds (32M/16M)

Choosing a different sampling rate

5M samples, @1MHz : will gather 5M samples total with a rate of 1M sample per second. That means that the capture duration will be (5/1)=5 seconds (5M/1M)