Jerry Whittle from Belleville Illinois on 9/1/2020 3:48:35 PM:
Depending on your weigh, 28s or 32s should be fine. I usually ride on 32s at 70 psi. I weigh around 215 lbs.
To be honest, I'd rather not ride such a nice bike as the Roubaix on the Katy Trail. If it's dry, that crushed limestone dust can get everywhere. If it's wet, the dust can stick everywhere.
I have fenders on my old steel touring bike, my gravel bike, and an aluminum tandem that I use for the Katy trail. After rides on the Katy, my bike and body are much cleaner than those without fenders especially if wet. I seldom take my nice Giant Defy Advance on the Katy except for short stretches to avoid busy roads.