This is already answered here, under the title 'How to change the style/width of cells in JupyterLab?'; however, you might overlook it unless you know that the current JupyterLab and Notebook Version7+ share the same underlying tech.
Specifically
For example, to increase the width of cells using IPython you could use:
from IPython.core.display import display, HTML
display(HTML("<style>.jp-Cell { width: 110% !important; }</style>"))
Based on here & tested in Jupyter Notebook 7.0.1 just now.
Keep in mind there are other settings you can edit via a form for how the Notebook layout is. To get there go to the Jupyter Dashboard that has the URL that ends in /tree
(the Dashboard is also where you select files to open) and then under the Settings
menu click on 'Settings Editor
'. In the 'Settings Editor
' look for 'Notebook
' listed in the side panel and edit the settings in the form.
Tip: Because of the same underlying tech a lot of the tricks and approaches for JupyterLab work in Jupyter Notebook Version7+. Meaning that you'll want to also search for solutions for Jupyter Notebook Version7+ among recent posts about JupyterLab.