Calculating heat sink sizes correctly is a bit of a science, and will depend on things like airflow and the heat sink material. I'm sure there are guides online that can give a better explanation than I can, but as a first step you will at least want to calculate the power dissipation we are dealing with here:
As an example, I will use the part MBR6080. (a 60A forward current and 80V reverse voltage-rated schottky diode in a DO-5 package. You might also consider other diodes or package types if your application does not specifically require a stud-mounted diode.)
According to the trace curves, at 25A the forward voltage will be slightly under 0.7V, resulting in a power dissipation of 17.5 Watts-- which is quite a lot for just a single diode. If the application has a lower duty cycle-- e.g. if the device will only be powered for brief periods with enough time to cool down in between, or pulsed on and off so that it only conducts part of the time-- a smaller heatsink may be possible, but if this is going to be running a continuous 25A, you may need a reasonably-size heat sink with fins and possibly a bit of airflow.
Just out of curiosity, what is the application here?