Yes, you can enlarge a legend key in an Excel chart without increasing the size of the text. Here's how:

1. Click on the chart to select it.
2. Click on the legend to select it.
3. Right-click on one of the legend keys that you want to enlarge.
4. Click on "Format Legend" in the drop-down menu.
5. In the "Format Legend" pane that appears on the right-hand side of the screen, click on the "Marker Options" tab.
6. Under "Marker Options", you can adjust the size of the legend key by changing the value in the "Size" box. You can also change the shape of the legend key by selecting a different shape from the "Marker Type" drop-down menu.
7. Once you've made your changes, click "Close" to close the "Format Legend" pane.

That's it! Your legend key should now be larger without affecting the size of the text.