8.2.3 Selecting the size (strength) of the key

The ‘Number of bits’ input box allows you to choose the strength of the key PuTTYgen will generate.

• For RSA, 2048 bits should currently be sufficient for most purposes.
• For ECDSA, only 256, 384, and 521 bits are supported. (ECDSA offers equivalent security to RSA with smaller key sizes.)
• For Ed25519, the only valid size is 256 bits.