I don't know about viewfinder brightness, and that can be difficult to judge objectively in any case, but as for size the 5D has an eyepiece magnification of 0.71x, the D700 is 0.72x. So if there is any difference visible, the D700's viewfinder should appear very slightly larger than the 5D.
Note: the above doesn't take into account how much of the actual frame the viewfinders show.
>So difference between 1Ds III for example having x0.76 magnification means that it is quite bigger than D3 or D700?<
No, not quite a bit - just a little bit bigger. You can get a DK-17M eyepiece that gives you 1.2X magnification for 35.00 that makes the Nikon's quite bigger than that. I'm pretty sure that you could get a similar eyepiece for the Canon. Plus it isn't really relevant comparing a 1ds MKIII to a D700 - and the 5D is "quite" obsolete and due for an upgrade very soon.