what was the most common engine size
More than likely its a 302 in that year and model.
if you're in the US, unless it was a police/fleet model (which will have P43 in the VIN) it is a 302 with CFI/TBI injection. If it was a police model, you MAY have the 351 with a 7200 Variable Venturi carb.
Search Ford LTD Crown Victoria Questions
Ford LTD Crown Victoria Experts