I'm no expert, but like everything this kind of depends on how well you fit your road bike and/or what the geometry of each bike is. Generally, a more relaxed/upright position is used for cross. I assume you mean cyclocross?

Maybe. Maybe not. Depends on brands, measurements, stand over leg length etc.

I'm have a 56cm Specialized road bike and also have a 56cm Specialized Crux. The 54 Crux had more standover, but was too short both stack and reach for me. The standover on the 56 works for me.

Only "rule" is get the one that fits.

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Suffer Well.

I ride the same size CX and road bike. The only time it's common to size down is when going to a TT bike.

Having said that, the size difference between a 56 and 54 can be easily made up within the stem/seatpost lengths. May want to consider the crank length between the two though. I like to remain the same crankl length on all bikes (mtb excluded).

It depends. If your 56 road bike is already pretty small for you, a similarly sized cross bike may be fine. However, if the top tube is crushing your balls, you might want to size down for cross. The general rule of thumb is to size down for a cross bike, but it's not universal due to different frames having different stack and reach. I could fit a 54 road bike, but I ride a 52 which fits me perfectly due to my frame's geometry. So if I bought a new cross bike, I'd probably get a 52 as well.

In my case, I ride 56 road/tt cannondale, and 54 cx cannondale. The cx bike feels more comfy to me, more upright, and it's actually making me wonder if I've been using too big a road/tt frame the last ~17 years! I was told (for cdale anyway) that it's common to size down for cx. But I'm sure it varies from one manufacturer to another.

the whole size down thing is how they used to fit bikes but in general you want to get a setup just like your road bike with a little higher bars and perhaps a bit shorter stem. Many riders will run theri brakes up higher on the bars to force a little more upright position.

bike sizes can be strange though, I ride a 58.5 top tube on the road, but in a felt I have to use a size 57, others a 58 others are even a 60. Ridley has kind of strange sizing.

Just to be clear, I'm not saying you're wrong, but just to emphasize the point of "it always depends", I have a 56 Cervelo S1 and a 56 Cervelo New P3. Even sizing down for a TT bike isn't a rule, just relatively common.

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The point is, ladies and gentleman, that speed, for lack of a better word, is good. Speed is right, Speed works. Speed clarifies, cuts through, and captures the essence of the evolutionary spirit.

It all depends on the actual geometry of the cross bike. Generally, look for a roughly similar stack and reach as your road bike. Many have slightly forward (from road) positions and a shorter stem, but not everyone subscribes to that fit philosophy.

For example, my fit on the cross bike has a little less stem length, higher bars, and a little less saddle setback compared to my road position, but not enough to account for a different size frame. I probably have a centimeter or so of less reach from saddle to bars from the road- a tighter, slightly more forward cockpit.

Some manufacturers have very different sizing due to some earlier preferences to higher bottom brackets and top tubes with relatively low head tubes. Ridley being a case for this classic style of European cross bike. As a result, many did size down. Recently, cross bike geos are more roadish, but still maintain the extended wheelbase and slacker head tubes for tire clearance and handling stability reqs for off road riding.

So, just check the geo table of the bike you are looking at or ask your fitter and the right size will be clear enough depending on which model(s) you are looking at.

j