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.

The reason for including this data inside the RSA encryption is that in
that position it doesn't increase the overall packet size.