How do I adjust the upper half of my columns to be a different width than the lower half

For columns, how can i adjust so that the first 10 rows of a spreadsheet with have one ABCD column width (this is a table, the data set is pretty short so I'd like to have the column width be narrow), and that the next 10 rows will have another ABCD column width (this is another table...but that data is more spread out so I want to have these column widths to be quite wide)

Thanks

The columns in Excel cannot be of varying width down the page.
The one thing you can do is for all your rows beyond row 10, for example; select cells A11 - E11 and then right click them. Select "Format Cells" from the menu list and when the pop-up appears, select the 2nd tab "alignment". In this alignment tab, put a tick in the "merge cells" checkbox and click OK.
You will now have what appears to be a single cell which is 5 cells wide.