16" tires few good tires on the market, there are two or three high pressure tires, few shops stock "good" 16" tires. Dual 16" bikes need large chain wheels, or special drive trains. Smaller tires offer a slightly ruffer ride.
20" Many more "good" tires on the market, dual 20" bikes need large chain wheels, or special drive trains. There are quite a few very nice dual 20" bents on the market.
20"/26" Can use standard gearing, many good 26" road tire to pick from
There are no hard rules on how a recumbent is built, or what is should look like. One builder may build a 16"/26" another a 20"/700 or dual 20's... whatever they think is better. Each has its negatives, and each has its pluses. The most common is a bike built 20"/26"
"When the spirits are low, when the day appears dark, when work becomes monotonous, when hope hardly seems worth having, just mount a bicycle and go out for a spin down the road, without thought on anything but the ride you are taking." - Arthur Conan Doyle