26" is the most sensible size for global touring.
The most common wheel globally is probably British 28" (Westwood rim) as used on the Chinese Flying Pidgeon and old British roadsters and their local variants throughout India and Africa.
Its not a great size to tour on these days but in the past it was the one people used.
The first off-road (ie no roads) crossing of
Iceland was done on this type of bike in the 1930s.