The entry fields have a limit of 254. This has nothing to do with the command line length of your compiled .exe.
Create a runtime variable, e.g. %MY_COMMANDLINE% and then you can use a command line value of up to 1,024 bytes.
BTW, if you can only pass one value then you are doing something wrong. See attached screenshot.
The following command line passes two compiler variable values to the compiler:
sb7.exe" /C "c:\sbtest\galedon.sb7" /CV TEST1 "AAAAAAAAAA" /CV TEST2 "BBBBBBBBBB"
BTW, make sure you have defined the compiler variables in the Compiler Variables Visualizer!
Hope this helps.
Friedrich