Renault-Nissan - Arty
Am I right in thinking that Nissan is the largest of the two?

Nissan appears to have greater world market penetration especially in the US and Asia.
Renault-Nissan - Aprilia
Yes indeed.

Nissan has a major design centre in the US and manufacturing plants in a number of countries, including the UK.
Nissan (and their premium brand 'Infiniti') sell well in the US, Canada, Australasia, Asia and Africa.

Renault's market penetration outside of Europe and some ex-French colonial African countries is tiny.
Renault-Nissan - Edd
Renault is buying Nissan it owns a fair % of shares in it at the moment and wants to use each other advanatges to its best ie use the nissan delaerships to sell Renaults and use Nissans presence in the US to sell Renaults in a few yrs time.