The RSA encryption occupies space equal to the modulus which is
from 128 bytes for 1024-bit to 512 bytes for 4096-bit.

Data encrypted under RSA with the larger keys is (with sizes in bytes):
24    3deskey (unchanged from older software)
64    hmac_key for HMAC-SHA256
32    hmac(2*512 of later header data) to prevent tagging
32    hmac(body)
32    hmac(328block) current header data a.k.a TTE standing for three,two,eight
32    aes_pre_key

The aes_pre_key is used together with HMAC-SHA256 to generate the 3
AES keys for the body, future headers and the current header data of 
size 328.