Since we usually spot and stalk them as they com ein from the fields in the early morning, we judge the overall size by which strand of the barbed wire they come up to... a "second strand" pig is usually a good-sized one, and a lot over that scond strand means it's a big one. (They usually go through the fence between the second and third strands, and it doesn't slow them down much. You can often hear the wire "singing" before daylight as the pigs slip between the strands.)
As far as whether they are fat or not, I just look at the body contours to see if the belly is tucked up high or nicely rounded. In general for an eating pig, I like to shoot a smaller hog in the spring when they've been eating grain. They tend to be fatter then, compared to a late summer or fall pig that's been eating acorns and roots, bugs, etc.