how to generate a public key ?

I'am working on my client api and they have requested a public key with the following points

1-Encrypt / Decrypt algorithm is: sha256.

2 -public / private key size should be 5000 bits or more (signature algorithm sha256).

3-Padding is RSA_PKCS1_PADDING

i have tried "ssh-keygen" and send them the output 

but they reply thats not a valid public key 

ssh-keygen -t rsa

 how can i acheive this public key with these points 

thanks