70 x 40 will work fine for most plane sizes.
What I found when I built mine was not so much the size of the top (I got hold of a very large top by going to the local factory that makes Melamine benchtops and buying a reject - it is around 160" x 48") but the height of the bench. I started out at 900mm (~3ft) high and son was cutting the legs down so I could work more comfortably on the planes. Even now at 750mm (about 2' 6") high it is still too high to comfortably work on the 42% plane that I currently have on it.