I imagine with the 5 new star systems coming in the .7a update, the game will have an increased memory for all the new things it will have to calculate.
Otherwise the way i have been adding memory is just editing the starsector.bat file directly and opening with that. Its a bit weird, but i'm sure when the game is farther along in development it will be easier to do. At the moment it might not be a good use of time to put in something that allows you to change memory usage if the memory usage can change drastically through future updates.
Looking at .7a again, the 5 new star systems almost double the current 6, and the persistent officers will probably add to it also. I couldn't imagine the game running too well without a little bit more memory, i have to at least double the memory SS uses with all of the SS+ mods i have