I am a beginner and designing my first UI wireframes for a large website with 80+ screens. My question is we set a margin width for the pages of 100px on each side of the page (the screen sizes are 1920px in total width). Is it good practice to stick to this margin throughout the project? Many of the pages contain a side menu and detailed forms and it would make more sense to reduce the margins on these particular pages for more room regarding the form data. So the question is: is it ok to have different margins on some screens of the website? Thanks
I think you are thinking of a design intended for paper rather than for a website. If before starting to create the website you define a margin of 100 px at each border, you will soon find functionality problems in the design adapted to a smartphone or tablet.
Personally when I start a project I usually define the preferred maximum width of the work area instead of the margins: for wider margins, narrower work area. These margins will be relegated to larger screen widths or disappear on mobile devices.
Picture from www.myebox.com
Consistency and adhering to standards are an important web usability heuristic, and it is an important practice to have consistent margins throughout a website. There are cases when you should balance consistency with exceptions to the rule, though it sounds like the problem you're describing might be more about helping the designer find more room in a layout, which isn't really considered a valid reason to break consistency.
100px is a fairly wide margin to impose on both left and right sides, as many users aren't using 1920px wide viewports. Some of your users with vision problems might need a much lower resolution, and your website should scale elements accordingly. If your layout needs more room in several form cases, you should look at adjusting the overall margins. Please consider using flexible widths vs. pixel values - here's why.