INT. A PUB - DAY
3 lads are sat round with pints. Lad3 looks a little worse for ware.
LAD1
What's wrong with you then?
LAD3
I've been crushed, that's what's wrong
LAD1
Crushed? How?
LAD3
That fat bird I pulled last night. Bloody hell, I nearly popped
LAD2
Fat bird? She wasn't fat at all, she was gorgeous!
LAD3
Remember how you said her dress looked a bit like the tardis...
?
I'm with you on this one, but there are a few on here who will be wondering what's going on. good effort
Is the joke that she's bigger on the inside of her dress than she appears on the outside?
Yeah, but I guess it's expected with this kind of sketch. It doesn't always hit you right away.
Yeah it is Lee.
Chris this is a not bad bit of dialogue from a sitcom, it isn't really a sketch.
I think you need to look at your structuring.
I was pondering the structure of this one for a bit, and I'll keep it in mind as some dialogue as I'm planning to embark on some sitcom writing soon.
Cheers for the comments.
It kind of falls apart with the line "Remember you said her dress looked a bit like the Tardis"
It doesn't make sense. Your audience will immediately think "How can a dress look like a Tardis?"
I think there's something in the idea though. Buggered if I know what. 
Yeah I was struggling to come up with some way to work it into something.
It all started from a line which popped into my head one day:
"Tardis is very slimming"