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.