As a Gold member they will in many cases have the paperwork printed and car already assigned for you, yes. In a large enough Gold location (most sizable airports, some large city locations) they will have Gold Canopy service with the name boards where you walk directly to your already assigned car. And although many frequent renters will dismay at this there are a few cases still, especially at a small location, when just being Gold may mean they will assign you the larger car before someone else (who might just be getting an insurance replacement or whatever.)

Obviously start by booking Economy, the smallest class.

At a small location, call them a few hours before, or the night before if you have an early AM, and see if they will set aside whatever most favored (to you) small car they have on hand.

If you rent from the SAME smaller location a lot, go further by introducing yourself to the manager/staff and letting them know your preferences directly. This probably won't work as well in a larger location as too many different people will be involved in assigning the cars, but in a small office they'll recognize your name.

At a large location calling probably won't be of much use. When you have Gold Canopy, you may have to settle for going to see what car is there first, then going back to the counter to see if they'll put you in a smaller one if you don't like what is there. There should be a separate Gold counter to help you; you don't have to go through the regular counter. The notes in your Hertz record can work some of the time but not all the time. It can't hurt really I suppose.

When I want to switch out of something already assigned to me that is an upgrade or just whatever vehicle the folks in the office think is the nicer car (lower miles, has the sat radio active, whatever), I always thank them for thinking of me and just note that such and such other car seems more appealing today or whatever. In your case you just tell them that although you appreciate the upgrade you really prefer the car to be as small as possible.