The size (length) of a key determines the strength of a key pair. The longer the key, the more difficult it is for any brute force approach to crack the encryption. The default is 2048 bits.
You can specify an encryption key of up to 4096 bits. The generation of a longer key takes significantly longer. However, the application of a longer key (after it is generated) degrades linearly relative to length of a key.
For all practical purposes, 2048 is a suitable length for most applications.