Unfortunately, the size of the legend keys is tied to the font size.  You can
change the color, but there is not a way to change just the size of the
legend key.

Do you want the legend keys to look bigger compared to the labels, or
only compared to the pie? If the latter, consider shrinking the pie
chart itself. If the former, that's not an option in Excel,
unfortunately: the legend keys are automatically scaled to the font of
the largest legend label. You can try to decrease the visual impact of a
large font by choosing one that's thin and greying it down a bit, or you
can choose a font colour of "white" (thus making it invisible), then put
a dummy legend label as a text box next to the big key blocks.

Or, you can introduce a dummy data point with a value of zero, so it
doesn't show in the pie chart. Then give it no border and no pattern so
it doesn't show as a legend key. Finally, click on the legend label and
make *that label only* large, then give it a white font colour, so
neither legend key nor label is visible, but the others are.

You'd better turn the legend border off though, or there'll be an
obvious gap. Also, the labels will be bottom aligned, and you can't
centre align them. You can mitigate it by ticking the "superscript" box
in the font dialogue, though.