Need help!!...big prints!!!
From the standpoint of any adjustment in shooting for an anticipated huge enlargement, it should go without saying that it is all about resolution, information density, and sharpness since any image softness is going to be magnified hugely by xtreme enlargement. The best technical enlargement result is going to come from choice of camera, and simple MP size is not the whole story, lens resolution.....and then technique to maximize what the hardware of capable of.
If you are used to only shooting for internet/web or small pictures you might not know where the maximum sharpness settings are for your particular camera(s) and lens(es). If you have been shooting for relatively big enlargements, maybe up to 16 x 20 then you should already be aware of what it takes to get the best resolution out of your equipment.
Personally, I don't really feel I KNOW a file/image until I have printed it out large. If it won't hold up to 16 x 20 then it won't go any bigger (my opinion) and can only do smaller formats.
One assumes that YOU are not doing the printing...or you wouldn't have to ask the question. Talk to the printing site as to what equip they are using and best resolutions /settings in the file you send them.
You will get various answers as to the issue of "how big can an xxMP file be printed". My biggest enlargements so far involve a Nikon D2x....12.2 MP file....blown up into 4 16 x 20" panels wide by 3 panels high.....that translates into about 6 1/2 feet wide. The native print resolution came out to something like 65 ppi. I was blown away by the resolution/detail and to how well it works as a mural. In your case, at a wedding, everyone is disposed to be positive, accepting, and gracious, so no one is going to get eyeball to print and register complaints. Not all cameras and not all pixels are the same so what worked in this case is not a universal limit.
If I KNEW I was shooting for max enlargement then I would stick with my best resolution camera and lens, use a tripod or monpod as much as possible, and fuss over the details of focus, sharpest f-stop for your lens, lowest ISO, etc.. IF you don't know what I am talking about then this is going to be a painful learning curve for you....and maybe your couple.