but now i must add and in a new "if" statement to the bottom of each variable selection list. How do i do this?.
Not sure what the problem is - you do it the same way as you had already added the other 13 if-statements!? You put the cursor at the end of the last program line and press Shift-Enter to create a new line. Then you type "if" (without the quotes) and immediately press Ctrl-J or you use the menu to insert an additional if-statement.
To avoid that long list if if's you might have a look at Primes various "lookup" functions. Especially "vlookup" seems to come quite handy for your problem - let Prime lookup the name in the first column of your table and return the appropriate value in the Area- or diameter-column.
https://support.ptc.com/help/mathcad/r6.0/en/index.html#page/PTC_Mathcad_Help%2Flookup_functions.html%23