We are trying to replace our bassinet mattress as I we last used it 4 years ago with our first child. Our bassinett is the Tutti Bambini Cozee Air bassinet (2020). Unfortunately the model we have they don’t make anymore and we are struggling to get a new mattress. I have found one that is a couple of centimetres larger that should be compatible. Our original mattress was 81.5cm by 50cm I believe. The new one I have seen online is 83cm by 52cm. My question is - how exact does the measurement have to be? I understand it can’t be too small because of gaps, etc. But if we can’t get an exact match then do we need to get a whole new bassinet? Many thanks.

Does the original mattress need replacing?
If you are happy that it remains firm, clean, well-fitting & flat, then it can still be used.
https://rednose.org.au/article/is-it-safe-to-use-a-second-hand-mattress
How has it been stored over the last 4 years?
Has the mattress remained firm & clean?

Buying a new mattress for an older bassinet can be difficult, as there are so many different types & sizes of bassinets.
Concern with the mattress size is the risk of gaps if it is too small, but also if the mattress is too large it may not fit flat which may lead to increasing risk of baby accidentally rolling to side/tummy.
A 2cm difference may elevate the sides/ends of the mattress. It’s hard to say without seeing the fit in the bassinet.

Another option is to place baby in a cot straight from birth.

Have a look at Red Nose & Product Safety Australia information about bassinets, which includes info about the mattress.
https://rednose.org.au/article/bassinets
https://www.productsafety.gov.au/consumers/keep-baby-safe/settle-baby-to-sleep-safely/bassinets-and-cradles-guide