Luggage capacity is a tricky one, as car models vary within categories as well as between categories. There is not a bug difference in size between the Intermediate and Standard. The Standard usually has more leg room. Luggage capacity is not a factor in the car category ratings. For example, the Full Size 7 seater SUV typically has barely any luggage capacity compared to the smaller SUVs (since the space is taken up if using the rear row of seats).

The safest thing may be to book the Standard size, then when you get to the parking lot, check out all the cars in that category. Also look at the Intermediate SUVs and if one of those happens to have more luggage capacity, then take an Intermediate since there is no charge for taking a lower category. But note, on that basis you would not then be able to claim back the difference in cost between the 2 categories since you will have chosen to take the lower category car.

I see you did email us with the same question, but I replied on here instead since this topic may be useful to others.