Drop-down list

The built-in Data Validation drop-down list has a fixed maximum number of rows it will display. In your version of Excel (Office 365), that limit is 12 rows. Because your list contains 15 items, Excel will automatically add a scroll bar to the list. Unfortunately, adjusting the cell height, row height, or font size does not change this behavior; the pop-up window size is controlled internally by Excel.

The Working Solution: Combo Box (Form Control)
To display all 15 rows at once without scrolling, the best and most reliable method is to use a Combo Box Form Control. This is a small object you place on the worksheet that gives you full control over how many rows are shown.

Here are the simple steps to set it up:

 1. Enable the Developer Tab: Right-click anywhere on the gray Ribbon at the top of Excel and select Customize the Ribbon. In the right-hand panel, check the box next to Developer and click OK.

 2. Insert the Combo Box: Click the Developer tab at the top, then click Insert. Under the section labeled Form Controls, click the Combo Box icon (it looks like a small dropdown box).

 3. Draw the Box: Click and drag on your worksheet to draw the box. You can resize it at any time by dragging the small circles on its border.

 4. Configure the Box:

• Right-click the Combo Box and select Format Control.
• Go to the Control tab.
• Input range: Highlight the cells containing your 15-item list (e.g., $A$1:$A$15).
• Drop down lines: Type 15 in this field. This is the key setting that forces the list to show all 15 items without a scrollbar.
• Cell link: Click the box and select any blank cell (e.g., D1). This cell will store the numeric position of the selected item (1–15).
• Click OK.

 5. Display the Selected Value (Critical Step):

• The "Cell link" (e.g., D1) now contains a number (1–15), but you need the actual value from the list.
• In a separate cell (e.g., E1), enter the formula:
  =INDEX($A$1:$A$15, D1)
  This converts the numeric code in D1 to the corresponding item from your list (e.g., if D1=3, the formula returns the 3rd item in A1:A15).

When you click the arrow on this Combo Box, you will see all 15 items displayed neatly without any scrolling required.

I hope this provides a clear and workable path forward! 

May be you can't extend visible part of dropdown box in Excel cell ranges.