Convection using any large rad will work, but what you have to realise is it wont work without some significat "temperature differential" factor. In other words you'll see higher temps. Now the rad design efficiency will affect this along with many other real world specifics, but without a decent air current, (fan or another form of artifically created convection going on you'll see coolant temps rise until the convection losses equal the heat input before it stablises. It would be very hard to pre-calculate exactly what temperature this would be on paper. I'm using a large JCB tractor rad, (about 20" X 20" X 2.5" with a 16" Kenlow fan @ 3v and it keeps my coolant at 2C to 3C above room temp, (at 15v it will keep near as damn it to room ambient, but everything ends up on the other side of the room, and my ears start to bleed
). If I turn the fan off the coolant will get to around 8C above ambient eventually before stablising.
Here's the rad and fan (I included small items, cd rom & my PC for size indication
):-
Another topic you may want to look
HERE at as it looks into ways of creating artificial convection without fans, although for a different reason..