In Civit you only have 3 options under the create tab. I assume you refer to those and somehow 2 don't show up.
If you want to use other aspect ratios you would need to run it on your own system with something like Automatic1111 or find another similar service.
You should be aware that these aspect ratios are some of the most stable for many models due to how they are trained in buckets. Unless you want to go ultra landscape or need a specific ratio and are willing to go the extra mile, stick to the existing ratios.