Donald Trump 's comparing sizes again -- crowd size that is -- 'cause he says he brings out bigger crowds than some of history's greatest orators ...

including Martin Luther King Jr . The former President of the United States spoke with reporters at Mar-A-Lago Thursday ..

. and 45 couldn't resist boasting about how massive his rally was on January 6, 2021 -- before the riot at the Capitol. Trump says people regularly claim the crowd was smaller than it really was .

.. saying he thinks a million attendees were there for him .

.. saying when you compare photos of January 6 with pics of MLK's "I Have a Dream" speech, he's got the civil-rights leader beat.

He tells the reporters to compare a pic from his speech to one from MLK's on the steps of the Lincoln Memorial -- he says you'll see the two crowds are the same. Worth noting ..

. Trump says they spoke at the same venue -- they did not. MLK spoke on the Lincoln Memorial steps, and Trump spoke at the Ellipse, about a mile away.

DJT says he's happy to let people keep saying MLK brought out more people 'cause he likes him ...

but, he certainly ain't buying it. Of course, Dr. King died in 1968 -- so, it's a little odd he feels the need to play the "mine's bigger than yours" game with him.

Trump's size debate is the second he's been embroiled in recently ...

just yesterday Kamala Harris ' team posted to Truth Social , shading Trump's crowd by comparing it to her own. #BREAKING : President Trump agrees to 3 presidential debates against K.