Here's how you might do it. In the middle of your line of text insert the underoverscript box from the Basic Math Assistant palette. Put a cap sigma in the middle box. From the Format drop box choose Size and then click on 24 Point. Then type normal size i=1 in the subscript box and n in the superscript box. And then italic f in the next space. I can't reproduce the output but pasting the cell gives
text
\!(*UnderoverscriptBox[
StyleBox["[CapitalSigma]",
FontSize->24,
FontWeight->"Plain"], (i = 1), (n)]) f more text
