Something like width=300
does the trick.
# Figure layout
fig.update_layout(template='simple_white', width=300, height=500, title='Main Title', yaxis_title='Distance moved',
legend=dict(title='', itemclick='toggle', itemsizing='constant', traceorder='normal',
bgcolor='rgba(0,0,0,0)', x=1),
xaxis=dict(title='This is a title', showticklabels=True, ticks='outside')
)