[OnTheRivet]

I'm 179cm, riding a 575 ETT compact road frame with 100 mm stem. I'm choosing a CX bike from two sizes: 55cm/ETT 550 mm and 58cm/ ETT 571mm. People keep telling me that a 58cm frame is too large for me for CX. I've never ridden a cx bike and there is no possibility to have any test ride.

Your road bike fit points to some oddness about your body morphology or just poor fit as at your height a 100mm stem is too short. It looks like you should really be on a 56TT with a 110or 120 stem on the road bike so I'd imagine the 55cm TT bike wwith a 110 stem would be ballpark if you can get the stem high enough without a crapload of spacers.