You might use a calculation field with a case function:
Case ( field 1 = "4x5" ; "S" ;
field 1 = "5x7" ; "M" ;
Field 1 = "8x10" ; "L" )
And I'd add a third field with two values: Glass, Ntrocellulose for selecting the type of material used for the Negative so that field 1 only has 3 values, not 6.
Thank you PhilModJunk. I took all your advice (adding a 3rd field with the two values) and put the calculation in as a case function and it was a simple and beautiful fix. Well done and much appreciated.