From Xojo Documentation

Property (As Double )
aiOSSegmentedControlItem.Width = newDoubleValue
DoubleValue = aiOSSegmentedControlItem.Width

Supported on Mobile (iOS).

The width of the item in points. Leave at 0 to have the segment use the available space. If the widths of all segments are 0, then the space will be divided equally amongst the segments.

Sample Code

Change the first segment width:

MySegmentedControl.Item(0).Width = 50