Rectangular buffer

I find your question a little vague because it is just titled "Rectangular Buffer". @Paul's answer would work well if you want to have a rectangular grid covering your site.

However, if you have a point layer and want to create rectangles, i.e. rectangular buffers around the points, there is a tool to do that. You can read more about it on their blog.

I also thought of another workaround. You could create ellipses around your points using the Table to Ellipse tool. Your major and minor axes would be 120 and 100 respectively. Then, you could use the Minimum Bounding Geometry tool to create the rectangles. The "rectangle by area" or "rectangle by width" should give you the same results.

One final workaround is to modify the script at the bottom of this ArcGIS help page to suit your needs. You'll need to add an additional variable to get the second buffer distance.

I think Create Fishnet might be for that you are looking. The tool can be used to create rectangular polygons or polylines of set dimensions.

You would probably want to set the number of rows and columns to 0 so that the number of rows and columns will be determined by the width/height.

If you want a more in-depth explanation of Create Fishnet (the 11 parameters - of which only 4 are optional - can be overwhelming), look to How Create Fishnet Works.

Your process can depend on the software you are using. An approach you can use is to generate a point file for your locations (GPS location in the field). To make the rectangle you can use the QGIS plugin rectangle, diamond, oval. This allows your to input a length/width that of your rectangle (equal to 100x120m). You will add attribute information to the point attribute table