You'll see left I ride a 26" tire cargo bike. My legs are short, 28" inseam; that is the frame for small people. Yubabike no longer sells the bodaboda, as they sell mostly to parents installing a car child seat. The risk of the bike falling over with 26" while loading a child is too great. OTOH I carry groceries ,ag supplies, & goods in boxes, so if those get dumped as the bike falls over being loaded, oh well. I have a 2 leg stand, which reduces but doesn't eliminate the risk of fallover. I find 26"x2" tire rides okay on our *****y pavement without a suspension. I would find 20" tires jaw rattling. However lots of people in high tax cities like the 20" versions. Our potholes last a year sometimes. Following a car through a light in traffic or riding in the rain I can't always miss the potholes. Plus the city has decided the standard driveway entrance is a 2" high ledge.
Two intermediate size tires are available on cargo bikes. 24"x2" comes on the blix packa. And if you are never going to ride your bike past 2000 miles, radwagen sells a 22"x2" version, that you can only buy replacements from the bike manufacturer. Replacement tubes, too, buy from rad or stay home. Rad's 13 pages of complaints about stretched loose spokes & cracked wheels are enough reason for me to not buy one. I wear out a pair of $26 tires every year @ ~2000 miles. I can buy 26" or 20" tires at the LBS; tubes at the grocery store.
Haven't seen any 27.5 or 700 mm wheel cargo bikes. See this category for the various models. .