The required width of the bridge depends on the curvature of your track and what you will be running on it. I would use NMRA recommendations to determine what you need.
https://nmra.org/sites/default...rances_july_2017.pdf
I have a few scenic express girder bridges similar to the one in your photo. The over slung bridge needed plenty of clearance. The underslung bridge didn’t need any, but I had to raise the level once or twice so I could run tall cars under it. BTW, I don’t think girder bridges are curved in real life, but it’s my railroad so…
