a bit of input from the female perspective.
unless your girl is into cuckolding and likes humiliating you, all comments or questions regarding her ex's cock will make her think less of you. it reads as insecurity, antagonism and distrust. none of which casts you in a flattering light. you have a right to be upset or curious, but as a grown ass man you have the responsibility to deal with your issues and not put that burden on her shoulders.
a jump back to cuckolding and a possible fetish, if the idea of her getting fucked by his massive cock turns you, then tell her that, she might reacted badly or she might be willing to indulge your fetish with roleplay or stories. either way you have opened the door to communication. the possibility of resolving true angst and maybe exploring a kink without harassing the poor woman endlessly.
if cuckolding isn't your kink, then your issue isn't really with his cock, your issue is your own self worth and self opinion. a little inventory of why you feel what you feel might help you in the long run.
ml
a woman who loves you will put up with no end of stupidity, you are going to run into problems when she decides loving you ain't worth the hassle.