I want to know if there's a way to change the size of newform.aspx, by default it looks narrow, but my design states that I need more space to display my form without having to scroll right or left, is there a way to do that?
Yes you can. At least in SharePoint 2013 on-prem
First create a new custom form. Go to the list where you want the new form and edit the form in SharePoint Designer.
Then create a new form (1), give it a name (2), select which kind of form (3), option to set it als default (4) and click ok (5).
Then open the newly created form in advanced mode (right click on the new form):
In my environment it is line 24 where you can edit / change the width of the form.
Hope it helped. Also posted the guide in my blog.