Philodendron xanadu - pot size for this indoor beauty?

LadyCoconut78

Hello,

I have a pretty advanced Philodendron Xanadu in my living room. It came in a 21cm pot and I've left it in that. I'm wondering if before the Winter I should repot it? Any signs to tell me it needs repotting? 

Currently it looks happy and healthy but I do think its a bit cramped looking (yet I know it likes this). 

Thanks.  

LadyCoconut78

Thank you Philippa for that. I've gotten so far on gut instinct so perhaps I should trust myself a little more.