When you change the size of a cell, you will necessary change the width of the complete column or the height of the complete row, or both. There is no way to change the size of a cell without affecting other cells.
If you want to change the width/height of a single row or column, just right click the row/column header, and select ‘column width’ or ‘row height’ as needed.
I don’t think you can set different page sizes in a single document. You can use the ‘page break’ preview to adjust the page breaks, and excel will adjust the text size as needed to comply with it, but the page size will be the same for the whole sheet.
You can also go to the topmost cell and “drag” the right line over to make that one cell column wider or on the leftmost cell drag the bottom line down to make that one cell row taller. But Carlos is right you can’t just change a single cell.
You can merge cells – effectively making the size of a single cell seem wider as in two columns or three columns wide.
Demo: Highlight cells b2-d2