You can set the width and height of widgets in guizero.

Widgets are sized by pixels or characters depending on the widget and what it contains.

from guizero import App, PushButton, Slide
app = App()

# a PushButton's size is noted in characters
button = PushButton(app)
button.width = 30
button.height = 5

# a Slider's size is noted in pixels
slider = Slider(app)
slider.width = 300
slider.height = 30  
Widget Characters or Pixels Notes
Box Pixels The size only takes affect if the Box is empty, if the Box contains any widgets it will size to the widgets inside it
ButtonGroup Characters The height of a ButtonGroup must divide by the number of buttons in it
CheckBox Characters
Combo Characters
ListBox Characters
Picture Pixels See Images for more information
PushButton Characters
PushButton with image Pixels PushButton's which have images are sized in pixels
Slider Pixels
Text Characters
TextBox Characters Only a TextBox's width can be set
Waffle Pixels