Hey Andy,
I painted a big old ldv van a while back, I had a little 50ltr one on loan and it was out of air after one pass down the van, so I hired a bigger one, the hire company screwed up and sent me a faulty one so I ended up with a 200ltr/15cfm monster and that was well over spec, I would say get the best you can, flow rate is king but a big tank helps!
;D