Aspect ratio simply refers to the shape of the image being projected. When the ratio of the projector, image, and screen match the image fills the entire screen and you will not see black masking bars.
An aspect ratio of 4:3 has the shape of a CRT TV, and a 16:9 has the shape of a flat screen TV. The 4:3 is very common for golf simulators because it allows you to go taller, giving you more room to swing, without needing a wider room. If space allows the 16:9 is great for 4K and HD images, but be aware if you are tall or have limited room depth that this option may be too short for you to swing a golf club. The 16:10 is an aspect ratio mostly used for computer displays and tablet computers. These days it’s somewhat uncommon for projection, but with some effort it should be possible to find a projector capable of this ratio.
For some customers, the projected image is secondary while the safety of the surrounding area is the priority. Selecting a size where the aspect ratio is non-standard, so that it fits your space, is also an option. Just be aware the image will not fill the entire screen if you are using it for projection and the black masking bars will be visible.
The 1:1 is a square and is not a standard aspect ratio for projectors, but may work if space is limited, although it increases the chances of seeing black bars and you will likely have to adjust the settings on your computer. Adjusting the settings on your computer may be difficult for most customers unless you are very technically savvy.