dining for six  

54" seems plenty big enough
for six, to me.

Depends on chairs
It totally depends on which chairs you use. 6 lightly scaled side chairs would fit fine around a 54" round table. 6 larger armchairs not so well.
We have a 60" table and it seats six very well and we use wider armchairs with it. It also depends how big your dining room is. 60" is a pretty big table, especially if you are working with a smaller space.

I have a small space. But I...
I have a small space. But I use Eames DCMs as my chairs. SO I think 54 would be sufficient. What about the different oval shapes? WHat are the general sizes you find in oval for a (vintage) Saarinen table?

Ovals are pretty big...
78"x48" and 96"x54".

Whitespike,
when I was doing catering, we put 8 folding chairs very comfortably around a 60" round. 9 fit perfectly. We could squish 10.
54" is more than enough for six people, even if you have big, fat chairs.

my kitchen
round is 42". only fits 4 but we only have morning coffee there.
dining is 84 x 42" slight oval. Nice for six but also accommodates 8
rather well. I made my own top to mock-up what might work.
Three years later...
60 does seem large. Not easy to reach the center without getting up?
An oval is a good use of smaller spaces especially if chairs slide
under.
I do like this Cherner oval.
http://www.nest-living.com/collections/cherner-chair/products/cherner-ov...