How do I change the line width in Sequence View?
Set to Wrap to Window
In Sequence view, if not already set, toggle on the "Wrap to Window" button to have SnapGene automatically wrap to fit the width of the window.

Set a Fixed Line Width
Hover over the "Fixed Line Width"button and use the dropdown to set a custom line width.
Use the drop-down to choose from a selection of preset widths, or click on the current value to edit and enter a custom width.
Choose the ∞ option to display the sequence unwrapped.

To change the default Fixed Line Width for new and imported sequences, see Set the Display Preferences for New and Imported DNA Sequences.