[tgflynn]

Also note that just specifying the number of variables may not be terribly informative if you don't also specify the maximum clause size and the number of clauses.

For example I see some instances here with 120 variables but they are 7-SAT with around 10k clauses. Unless I'm mistaken reducing those to <=3-SAT will require adding about 20k more variables and tripling the number of clauses.