Kamala Harris isn't too proud to compare sizes ...

roasting some unruly hecklers at her rally -- telling them she thinks they're looking for a former prez's much smaller crowd. The Vice President appeared at a rally in La Crosse, Wisconsin Thursday ..

. and, a few hecklers started yelling at the Democratic nominee for president as she spoke. Hard to hear what they're yelling, but KH quickly shuts them down .

.. telling them they're at the wrong rally -- which gets laughs and tons of applause from her audience.

VP Harris waves to the audience members heckling her and lets her supporters yuck it up ...

before adding "I think you meant to go to the smaller one down the street," to more rapturous applause. Of course, crowd size has been a contentious issue between the two candidates ..

. with Donald Trump claiming he pulls in much larger crowds than Kamala. The war of words got so big, even former President Barack Obama had to weigh in -- using NSFW hand gestures.

BTW ...

Kamala's making a joke here -- 'cause Trump was spotted earlier today in NYC and J.D. Vance has an event in Pennsylvania.

So, looks like Harris had Wisconsin all to herself. Wisconsin is a key battleground state in the upcoming election. President Joe Biden barely won its 10 electoral votes in 2020 and Trump won it in 2016 .

.. so, it's anyone's guess which way the state will lean this time.

They say size isn't everything ...

but, it seems to be pretty dang important in this election!.