Hey Lou!
I sold my First pretty quickly - love the shape, but I just don't get on with bags that don't have long straps... and unfortunately the Bal clasps are not easy to remove and replace like the Mulberry ones.
So I moved on to the Town (x 2! [emoji41]), but recently sold the gorgeous but too pale one, and decided to try the City S.
Haven't used it yet but having tried it on, I like the size - in Mulberry terms it's smaller all round than a Lexy, much less boxy than an SBS, and probably holds slightly less than an SDR. Will try to do a comparison for you!
Measurements are 30cm wide, 20cm high and 10cm deep (maximum, at the base - it tapers towards the top)