'Nah, not really. At most we compare sizes and tell dirty jokes.' 'Compare sizes of what?' 'You know... that.'
Literal
''No, to-such-extent not-is. At-most, size [object-を] compare [and-たり], dirty-talk do about-that is-[and-し]'' ''Size [quotative-って] what [possessive-の]?'' ''That-thing [possessive-の].''
A coy exchange about risqué topics. せいぜい ('at most, at best') sets up a minimization. ~たり ...する is the 'doing things like X' listing form. わい談 (わいだん) is a formal/slightly old-fashioned term for 'dirty/risqué talk' — the 猥 character means 'obscene, indecent'. The evasive ナニの ('of [something]') uses ナニ as a polite placeholder for something the speaker can't quite name — the 'whatsit' function.