Starting with a desired image height of 60 pixels on a sensor, we need to start with a particular sensor in mind - using my 7D Mark II as an example, the sensor is 15mm in height and produces an image 3648 pixels high - that's 243.2pixels/mm, which means your 60 pixel image covers about .25mm on the sensor, or about 1.67% of the total image height. That means that, in order for, say, a 1.8m tall human at 30-40m to map to that height on the sensor, the total vertical field of view will be about 108m, which at 30m distance implies an angle of view of about 74.5 degrees (or 69.7 degrees at 40m distance). That's the equivalent of about a 10mm lens for the 30m distance, or an 11mm lens for the 40m distance. Those are very wide-angle lenses. So, the bottom line is that you're going to have a hard time getting the images of the people that small at that distance. A more normal lens of say 24-50mm will give a bigger image of the person than you were asking for. If that's a problem, you can always scale the image down in post-processing, or use a lower-resolution mode of the camera, or something.

As far as depth of field goes (there are many online calculators or mobile apps out there for this), to get a 20m depth of field at 30m with a 50mm lens requires about f/1.4; at 40m it requires only f/1.0. A 60m depth of field would be either f/2.8 or f/1.8, respectively. So, even relatively slow f/4-5.6 lenses will give you plenty of depth of field.