I've never rented a car before. I want to rent through Costco, but also utilize my National Emerald Club Executive benefits (I have no idea why I'm Executive since I've never rented from any company before). However, National says to "simply reserve a Midsize car..", but Costco doesn't have a "midsize" category, just the following: economy, compact, intermediate, standard, full, etc. I'm not sure if I'm supposed to rent an Intermediate or Standard... (it'd be awesome if I could rent an Economy, but I doubt that).

Is it possible to use the Emerald Club benefits through Costco, and if so, which one do I reserve?

TIA