If I ride a 56 Cannondale road bike, should I size down for a Cannondale cyclocoss bike ? Local shop has some 2012 clearance CX in 56. I won't race cyclocross, just messy winter rides on pavement & dirt roads . (As if I need another bike )

Go down a size from road to CX is what I've heard used as a rule of thumb for a starting point. Only one way to find out though. Pics if you buy it.

A 56 Cannondale cross bike will fit almost exactly like a 58 road bike.

In fact, I ride a 56 Cannondale road bike, but their 52 cross bike fits me perfectly - a 54 would be too big for me. (Note, I am referring to the CX9 cross bike. The geometry on the CaadX may be different. IIRC, the geometries on most Cannondale road bikes, Caads 9 & 10, System 6, Super Six, Evo... are pretty much the same.)

It just depends, but yes in general you can normally size down. Sometimes. It gets confusing because some companies try and compensate by adjusting up so that a rider can ride the "same" size throughout their line - "m" for all as an example.

Cannondale is one of the ones where you normally have to size down.

Compare stack and reach for your current bike and the cross bike you're considering. Cross frames often have shorter head tubes, but this is offset by the taller fork. Also, cross frames often have steeper seat tube angles for fitting a bigger tire in shortish chainstays, so you may need more setback.

My cross and road setups are very similar in size. The main difference is I use compact bars on the cross bikes for a slightly more upright position in the drops. I don't like too deep a drop when I'm getting bounced around.

Match the top tube not the 'size' which is seat tube , tippy top of the seat tube..

Since you are not going to Race Cross, even a Hybrid will do... just will have Flat Bars.

and just take a test ride and see.. 1st hand..

I just looked up the standover height and it's about 2 cm more for CX, so I guess I'll pass.