For a control or for a panel ?
What's wrong with
ScrollBar_SetAttribute (panel,scrollbar,ATTR_SB_WIDTH,nuValue);
There's a really good example in
MeasurementStudio\cvi\samples\userint\custctrl\scrollbar.
Not so sure about thickness of panel scrollbar, SetPanelAttribute(panelID,ATTR_SCROLL_BAR_SIZE,variable) only seems to take VAL_SMALL_SCROLL_BARS or VAL_LARGE_SCROLL_BARS