I allow 60".

If you look at laundry rooms here on Houzz, most washers and dryers have a snug fit in their alotted space. IMO a lot of extra space around the W/D looks sloppy. I had 61" of space so I put a 6" drawer base in and the 54" W/D fit nicely in the remaining 56 inches -- and baseboards took up an inch of that 56". You just need to be sure of the measurements of your machines.

Find the dimensions of the W&D you have in mind, and add 3-inches (a inch on each side and an inch in the middle), for minimal dimensions. Or save yourself the effort and follow Mark's advice. I always allowed 60" too!

I have the large Miele W&D and they will fit in a 55" space. You want a bit more room because it isn't easy to pull them out if they are wedged in there.

The wider you make it the easier it will be to get that sock that falls off the top of the dryer.

So that's where all my socks have gone!

Don't forget the plumbing and vent connections in the rear...