Originally Posted by mstateglfr

The 52cm Smoothie has 17mm less stack and is 9mm longer reach than your Specialized Roubaix. It will stretch you out more, assuming you use all the same components.
The 51cm Smoothie ES has 2mm less stack and 1mm less reach than your Specialized Roubaix, which is nearly the same. Additionally, the BB drop, HTA, STA, and chainstay lengths are all almost identical to your Roubaix. Your Roubiax has 59mm of trail when using 25mm tires. The Smoothie ES with the no7+ fork and 32mm tires would have 65mm of trail. This would mean steering feels slightly less twitchy/fast with the ES. It would also mean the bike would feel slightly more stable at speed(because the fork would 'self correct' a little bit more and want to go straight the faster you ride). The difference is only 6mm of trail and that isnt massive, but it can be noticed.

You could make up for the feeling of being too stretched out by swapping the stock 90mm stem on your Roubaix to an 80mm stem on the Smoothie ES, and also swap the 75mm reach Roubaix drop bars for something with less reach like the Whiskey no7 6f bars that have 67mm of reach or the Zipp Service Course 70 with 70mm or reach.
https://whiskyparts.co/handlebars/no.7-6f-drop-bar
https://www.zipp.com/bars/service-course-sl-70/

Those changes would bring you back 15-18mm and since the Smoothie ES has a slightly slower steering feeling, a shorter stem wouldnt make it too twitchy...it would actually cancel a bit of the slower steering and make it closer to what you currently have.