With the needed background info out of the way, one way to get it done would be to use CInt to convert the result of fieldsize divided by 2 to an integer. From there add 1 to the result.
Here's an example:
AND RANKF03 <= CINT(FIELDSIZE / 2) + 1
Note: One way to test the above line for accuracy would be to TEMPORARILY add the following line to the expression:
AND FIELDSIZE = 8
From there, run it through the Data Window with the data broken out by SQL-F03 Rank. Since the entire results set will be limited to data for 8 horse fields only - when you see data for F03 ranks 1-5 only you will then know that "CINT(FIELDSIZE / 2) + 1" is working as intended.
Reminder: Don't forget to remove the temp line AND FIELDSIZE = 8 before saving the final UDM definition.