I'm having the same issue. I have a Macbook Pro 2018 with 2560x1600 retina display. After downloading the Sims 3 64-bit update it opened up with the 4:3 display in full screen so I tried to get the display to a resolution that fitted my aspect ratio better. However, the 2560x1600 option in the menu just made my display blurrier and I did command-enter to return to the window and that is where the display looked the clearest but the menu got very small. I personally like to play in full screen so I tried to do command-enter back to full screen and it got all blurry again. I can only play full screen in a low resolution.
@lillums23 Try switching to an intermediate resolution, not 2560x1600 but something else with a 16:10, while in windowed mode, then switching back to fullscreen. You may have to experiment a bit with different resolutions to get something that looks good.
Again though, it does seem that Sims 3 uses fewer pixels to take up more space on the computer screen than expected.
