I'm not very familiar with geometry
package, so I read instruction in its manual :-). There I find nice explanation, how to set up geometry
. As I mentioned in my comments, parameter marginparwidth
is no independent nor locally defined size. If you select its width properly, I will not disturb any other settings of page geometry. In MWE below I consider the following rule:
outer=marginparsep + marginparwidth + distance between marginpar and page border
The following MWE, where I anticipate twoside
option for your document, show an example for page setup. For more details, I strongly encourage you to read package documentation. It has only 18 pages with a lot of useful information.
\documentclass[12pt,twoside]{article}
\usepackage[a4paper,
inner=20mm,
outer=50mm,% = marginparsep + marginparwidth
% + 5mm (between marginpar and page border)
top=20mm,
bottom=25mm,
marginparsep=5mm,
marginparwidth=40mm,
showframe% for show your page design, normaly not used
]{geometry}
\begin{document}
\thispagestyle{empty}
test
\marginpar{margin notes (test)}
\end{document}
It gives the following layout of page:

For any further help, you need to give more information, what is your problem. Ma answer and given MWE is based on my guessing.