In reply to a remark from Marco Rubio during a Fox News debate, Trump said,

“Look at those hands, are they small hands? And, he referred to my hands — ‘if they’re small, something else must be small.’ I guarantee you there’s no problem. I guarantee.”

Time – Watch Trump Talk About His Private Parts at the Debate
CNN – Donald Trump defends size of his penis
Vox – Donald Trump just defended his penis size at the Republican debate