In Scale mode, click on a zsphere and drag to resize it.

Clicking and dragging a gray link-sphere resizes the whole chain.
Alt+click and drag to inflate or deflate the chain.
Macro for scaling ZSpheres to the Draw Size
This macro can be used to scale a ZSphere to the size set in the Draw Size slider. To use, save the txt file to the ZBrush 2019.1\ZStartup\Macros\Misc\ folder and restart ZBrush. There will then be a new button called ZSphere-DSize in the Macro>Macros>Misc sub-palette.
Press the button immediately after drawing a ZSphere. To resize a different ZSphere, turn on Scale mode and then click the ZSphere to select it before pressing the button. If Symmetry is on then symmetrical ZSpheres will be scaled.
- To download, right+click this link and choose “Save link as…” ZSphere-DSize macro