The term “largest” can be a bit confusing, since aircraft could be measured in different ways, wing span, length, cargo capacity, passenger load and so on.
For the purpose of this question, we’ll use maximum take-off weight as the measure of largest airplane still flying.
Using that measure, the Antonov An-225 would carry the current title as largest airplane. The maximum take-off weight is listed at 1.3 million pounds. A close second is the Airbus A-380 at 1.23 million pounds and the Boeing 747-400ER at 910,000 pounds.
The An-225 was designed to carry the Buran space shuttle developed by the former Soviet Union. It is now used as a cargo hauler.
Once the A-380 becomes operational, designer think future models may exceed the current weight of the An-225.
Thanks for asking,
EAA Young Eagles