This specifies the label width. The placeholder {Global.LabelWidth} can be used. This placeholder uses a global property set in the 'Label width' property in the 'Properties' pane. When specifying a set width use CSS syntax. e.g. 5in, 250px

Using the {Global.LabelWidth} placeholder
Global property in 'Properties' pane.
Resulting 1in button
This is only the case for field labels positioned to the right or left of a control. If positioned above or below a control the field label will be the width of the control.

Here are some examples using CSS syntax for general reference:

Width is 14cm
Width is 120px
Width is 100mm