I'm going to be loading some large caliber rounds, 470 NE,450-400NE and wondered what lube works best on these big cases? Do you full length size or neck size?

Imperial die wax.

Neck size and full length when required.

For a double or single?

+1 for Imperial die wax

I just set the F/L die to size my 404 cases to be an easy fit. Only ever lost the odd case from neck splits after not annealing often enough, never lost a case from F/L re-sizing.

BTW I use low melting point silver solder to repair the neck splits and carry on using these cases for cast loads which works fine, I just hate to waste NE brass.

This will be for a Merkel double rifle

Most die waxes work well with those thin rimmed cartridges..I like Imperial Die Wax best I suppose..

I size a double gun rounds to fall loosely into the chamber and be sure the bullets don't move from recoil..Best to use a tapered Lee crimp as the big case will crush very easy. I like a powder such as IMR-4831 that fills the case so the bullets have a stop to help them not set back, and I like the dies ever so slightly undersize. I turn the expander ball down a bit at at time to just barely snug up the hold on those thin cases, being careful not to turn them so much the bullet crushes the case with seating bullets, this is tricky but when done properly you will never have problems inn the field...load doubles with tender loving care, no slam banging the cases in the dies.

These big cases are not as hard to size as you think; they operate at low pressure and are fairly thin. I use Hornady wax; most of the big ones are easier to size than, say, a small magnum case.
And yes, you can't slather them with lube and force them into the dies; you will get wrinkles for sure. And some, like the 450, have thin rims; carefully milk them in.

Imperial! Just do it!

I use my own wax I made from bees wax and olive oil.I melted the pure bees wax and mixed it with some olive oil to soften.A tiny spec of the stuff will lube a 500NE case.No chemicals or chemical oils! I then rub the stuff off with a towel and if I want the case to shine like crazy I throw it in the tumbler.Just FL size and seat the bullet.No expander needed.Be carefull when seating the primer so as not to bend the rims.Make sure primers are in deep enough.Check rds before using by chambering each round individually.They should fall in the chambers easily with their rims flat against the chamber or breech face.

I use Lee paste lube.

But do lube as the .470/.500 case has a very minimal rim and you can pull one out of the shell-holder fairly easily leaving you with a stuck case to remove.

Did this with a .500 NE that I forgot to lube. Never stuck a case with Lee Paste.

Other than that...it is just a cartridge to reload.