I would like to know this as well. Im currently using 1 3/4 in pots for lettuce. Seems okay

Thanks for threads. I am also facing same query and eager to know as well. I am also using 1 1/2 in pot for my plants.

There is no standard. I've grown everything from lettuce and herbs to tomatoes in 2" net pots. It's really just a matter of preference. Unless you need the roots to be contained within the pot, there is not a lot of reason to use larger pots in my opinion. But I am sure others will disagree.

I don't have a lot of experience but I will throw in what I have learned. The small 1.5" net pots will basically only fit a piece of rockwool. If you want to add a medium to the net pot then I would suggest a 3.75" net pot or a little bigger.

I've grown squash in 5" net pots inside a 6" pvc pipe and tomatoes, peppers and eggplant in the 10" net pots inside 3 gallon buckets. Both ran drip for a few minutes each hour, not constant like nft so I needed more medium. The net pots had grodan and clay pellets. For things in nft or water culture I would use much smaller since the roots grow out the net pot more. For drip and ebb and flow a little bigger is better to hold the moisture. The medium you use makes a big difference too, rock wool holds way more moisture than perlite or clay. There is no set guidelines I've seen but the plant will let you know if it needs a bigger pot.