What's the price difference? Are you regular Emerald or Exec?

I've only reserved premium once with National, and was pretty much given my choice of anything I wanted (non-aisle location). Every other time I've made a reservation it's been for a midsize, including at the aforementioned location, and I've never ever driven anything less than a fullsize.

Short list, all on midsize reservations as standard Emerald:

2015 GMC Yukon SLT, ~12k miles
2014 Taurus Limited, ~16k miles
2015 Chevrolet Camaro Convertible V6
2015 Cadillac CTS 3.6
2014 Dodge Ram 1500 Crew 5.7 4x4
2015 Chevrolet Impala LT, new body style

If CAE is an aisle location, and there's a significant ($20+/day) price difference, then book midsize. Also, remember than if you book a premium and don't like the selection in the premium section, you can choose from the aisle and request that the booth agent/counter agent lower your rate to the midsize price.