Everyone has different philosophies, In my opinion:
I keep the saddle position the same between each bike, relative to the bottom bracket.


For CX: I like my stack and reach to my hand position to be ~2cm higher and closer from road to CX. I also run wider bars (44cm vs. 42).
For Gravel: I like my stack to be the same as road, but hand position 2cm closer. This keeps me relatively aerodynamic, but gives me a bit more arm movement for bike handling.

I can see an argument for a longer bike being more stable. If this is worthwhile to you, then you may be able to achieve that with a larger frame, presuming you can run a shorter stem to bring the bars back and make up the difference in stack height. This will center your weight between the wheels more, which is often beneficial for gravel riding and CX. (I hate short CX wheels bases as I tend to overweight the front tire on tight corners.)

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I ride a CX bike for gravel as I find most gravel way bikes way too cruisy. I like long and low and I like to go fast.
The 65mm BB drop is nice for when my gravel rides go into adventure territory... single track and gnarly bits.