If you want to check for the size of the party, paste the following into the script condition:
$game_party.actors.size == X
X can be replaced by the size of the party you are checking for.
If you want, instead of '==' you could use '>' (greater than), '<' (less than), '>=' (greater than or equal to), '<=' (less than or equal to) or '!=' (not equal to)
Hope that helps