Bend Arc will do what you want. Assuming you started with a line of text, centre the Gizmo on the text. Move the text so it is tangent to the cylinder wall. Looking straight down onto the cylinder (Y axis by default), switch to Bend Arc, click on the green cone at the centre of the bounding box (where the Gizmo centre was) and drag to create a circle of 180 degrees (hold shift for increments). Yes it will be big. Then click the white cone and drag towards zero and probably even negative (i.e. past the centre so it points in teh opposite direction) to shrink the size of the circle until it matches the size of the cylinder. You can flip between Gizmo and Bend Arc to adjust position as you go. When with Bend Arc hit Accept.
To simple shrink the circle text you already have in the image, centre the Gizmo on the circular text, click on the green scale Y icon on the Gizmo, hold ALT and drag. This will scale in the directions other than Y, i.e. X&Z. In other words, it will do a radial scale.
If you’re new to Zbrush then check out Michael Pavlovich’s Intro to Zbrush. An easy to search playlist covering all the basics.
Also Maxon (Pixologic) ZClassroom covers all the features but also has project based examples so you can see how it’s used to create various sculpts.
To see Zbrush in action browse the many live streams Zbrush Live
