Brother keeps asking me what I think of his penis and it is beginning to make me kind of uncomfortable

So, for context, I (F22) walked in on my brother (M18) jerking off the other day. I of course did what any sane person would do in that situation, which is hastily apologise and get the hell out of there!

Anyway, I thought that would be it. He would be super embarrassed that his sister walked in on him like that and would want to avoid bringing it up at all costs. Well, it turns out I was wrong…

Over the last few days, he’s turned into a bit of a creep. Asking me if I thought it was big etc. At first, I tried to brush it off with a nervous laugh, jokingly calling him a weirdo, but he has been unnervingly persistent and it’s kind of starting to freak me out.

The way I see it, he knows full well what size his penis is and he’s just looking for an ego stroke.

What should I do or say to get him to buzz off?