Pashley Picador weighs about 70lbs. Anything heavier is probably excessive. Industrial trikes are made for hauling big loads over flat surfaces.
In the UK we have these adult utility trikes but also
high performance trikes (not to be confused with recumbent trikes).
The performance models usually use std 26" or 700c wheels and are made from quality bike steel (butted chromoly)
The wheel size does not affect the height of the rider off the ground or visibility.
20vs24" seems to related to the size, in some models, 20" version is smaller all over.
Pashley distributors