OBaby Zezu Multi Mattress Size?

[Sahkoora]

Just bought an OBaby Zezu Multi and stupidly stored it in my dad's loft until nearer the time.

I want to buy a mattress for the pram bit so DS3 will have a bit more padding. Can anyone who has one tell me the size I would need to buy, and also what sort of shape?

Googling has only revealed the size and shape of the Zezu and I'm not sure if it's the same size as the Zezu Multi.

Thanks very much!