How much space needs to be above and beside baby's head in a bassinet? I have read the rest of the risks and guidelines but couldn't see this. Mattress is 70cm long. Baby is about 55cm long and 6.5 weeks old. Not rolling over or looking like rolling over.

There are no specific measurements for the head space around baby, but ensure there is room for baby to move head, there is adequate ventilation & no restraints or soft/padded material around bassinet.
Product Safety Australia also recommends the height of side should be at least 300mm.
https://www.productsafety.gov.au/products/babies-kids/kids-furniture/bassinettes

Red Nose has the following information about bassinets.
A bassinet should have good breathability zones on all four sides to reduce the risk of suffocation and to provide good ventilation for baby.
https://rednose.org.au/article/bassinets

Remember, that once baby is attempting to roll & looks like being successful, it is important to cease wrapping & move baby to cot.