The standard mesh size of the net for the sampling of phytoplankton and zooplankton is 64 micrometers. Rather than its depending upon the size of the plankton.

If you want to know what phytoplankton is in your system don’t use a net all. Phytoplankton can be as small as 1 μm.

Plankton net of 45 micrometer mesh could be effective. However, if the water is dominated by small sized group (Picoplankton) which is the smallest in size (2 micrometer in diameter), using net may not be advantageous.

A 64 micron mesh net is good for zooplankton, but using a tube sampler to collect water to twice the Secchi depth allows you to collect a well mixed whole-water sample in a jar (at least 400-500mL) that can be settled in graduated cylinders, so you have a condensed phytoplankton sample that picks up all sizes of phytoplankton. Let the sample settle for 7 days, then you can look at it under a microscope.