Default button width is automatic. This means that each button takes exactly as it needs to render design based on button style and counters.
Fixed width allows to set custom width of button in pixels and each button will take that width. Very popular and widely used in sidebars, post vertical float or widgets.
Full width buttons
Full width mode automatically calculate width of single button based on used to fit into full container.
Display in columns
Column display allows you automatically to split share buttons in columns. Very popular and widely used method in content sharing, fly in or pop up display.