That's too vague to answer. It's not the size, it's the weight and profile that would affect mileage. Even then there's no way to put a number on it. There's way to many variables.
Suffice it to say any load will make that V6 suck more gas then it does now.
When we towed a smallish U-Haul from Bakersfield to Houston with our V6 '95 2WD SR5, the mileage was 2-5 mpg worse, depending on where we were. The 2nd gen's were pretty gutless, so towing was a strain, especially climbing up to Flagstaff, Arizona.
