Which is the smallest available widebody on the market?
This question is broad enough to try out the new table feature:




Smallest by
All time
In production




MTOW
Airbus A310-200 (144t)
Boeing 767-300F (186.9t)


Length
Airbus A310-200 (46.66m)
Boeing 767-300F (54.94m)


Outside Width
Boeing 767 (5.03m)
Boeing 767 (5.03m)


max. Payload
Airbus A310-200 (32.8t)
Boeing 787-8 (43.3t)


Exit Limit
Airbus A310-200 (275)
Boeing 787-8 (381)


Passengers*
Boeing 767-200 (214)
Boeing 787-8 (242)


Seats Across*
Boeing 767 (7)
Airbus A330neo (8)




* number of seats for typical 2-class layout, seats across for typical economy
(source: Wikipedia)
In summary, the Airbus A310 was the smallest wide-body ever produced by most measures. The smallest wide-body still in production today is the Boeing 767, but only the freighter variant is still produced. The smallest passenger wide-body in production is the Boeing 787 Dreamliner.