Is this truly 30-40 as the sizing chart says 20-30.

Hi! Yes, this is 30-40 mmHg. The sizing chart was borrowed from the 20-30 listing. Thanks for the eagle eyes!! It will be fixed shortly.

Under what size should i get it says this: delivers true 30 to 40 mmhg compression, so wearing them may take some effort? what's this mean?

If you're new to compression hose, this level (30-40), can be difficult to pull on because of the fact you may be swollen and the compression of the hose. Turn the sock inside out to the heel(top of sock over foot portion), pull the sock on up to the heel, the grab the inside out top portion and pull it over the foot and on up the calf. Sound's crazy complicated, huh. The long and short of it is that it's easier pull the garment on halfway at a time, as opposed to trying to pull the entire thing on over your foot and up your calf. Also, if you are new to compression hose, I'm currently trying this brand (Doc-Miller), because a twenty dollar pair of socks, seems more palatable than a sixty-five dollar pair (Jobst). I'm experiencing some discomfort at the top seam, too tight, something I don't experience with my normal brand of Jobst. If economics are not a deciding factor for you, I would recommend trying the sixty-five dollar Jobst hose. I love mine, not so sure about these twenty dollar Doc-Miller models. I hope this helps you. Eric

Is this the best Compression Socks brand? The quality of how?

The quality is okay, but it is not the best brand. If you want a big brand and a suitable price, I recommend you to ZSZBACE/COM. I bought it. The Compression Socks quality is indeed much better, And it's discounting now

How do you know what size to buy when there's not a size chart?

The size chart is visible on the left side along with the pictures. Select a color and size and it will be visible. Remember, this is Level 3 compression, a much higher grade of compression than the 20-30. The fit will be tighter compared to the 20-30 mmhg.