There are no specific requirements. They must be on an accessible route and within appropriate reach ranges. This could mean the requirement of a compliant curb ramp or ramp depending on the circumstances. The accessible route must be a minimum of 36” wide and a 30×48 clear floor space is required at the accessible mail boxes.
Thank you for reaching out. While not ideal it could be possible to design a ramp that will comply with the ADA requirements. There is no explicit provision prohibiting a curved ramp. The specific issue will be getting the cross slope correct such that you don’t end up with a wheel off the ground.
There is no distance requirement for the mailboxes. It is possible to contact the USPS and request door delivery or other considerations to alleviate the issues you have with your disability.