You have to use the power consumption estimation tool, and then do that for say 80% gates used at the frequency you intend to use at to get an estimate of a max figure.
Yes, it's totally dependent upon the design and clock rate(s).
They do have a base static consumption, but that doesn't tell you anything.
Beware of traps like startup current, which can be much higher than your design consumption. So your PSU design has to be able to handle the brief power-on current too.
I don't recall if this is the case for the Spartan 6 though, but worth investigating.
All the manufacturers have very extensive apps documents on this.