Tips for a small system:

The following are set in options.h

- You can safely disable blowfish and twofish ciphers, and MD5 hmac, without
  affecting interoperability

- If you're compiling statically, you can turn off host lookups

- You can disable either password or public-key authentication, though note
  that the IETF draft states that pubkey authentication is required.

- Similarly with DSS and RSA, you can disable one of these if you know that
  all clients will be able to support a particular one. The IETF draft
  states that DSS is required, however you may prefer to use RSA. 
  DON'T disable either of these on systems where you aren't 100% sure about
  who will be connecting and what clients they will be using.

- Disabling the MOTD code and SFTP-SERVER may save a small amount of codesize

- You can disable x11, tcp and agent forwarding as desired. None of these are
  essential, although agent-forwarding is often useful even on firewall boxes.

If you are compiling statically, you may want to disable zlib, as it will use
a few tens of kB of binary-size (./configure --disable-zlib).

You can create a combined binary, see the file MULTI, which will put all
the functions into one binary, avoiding repeated code.

If you're compiling with gcc, you might want to look at gcc's options for
stripping unused code. The relevant vars to set before configure are:

LDFLAGS=-Wl,--gc-sections
CFLAGS="-ffunction-sections -fdata-sections"

You can also experiment with optimisation flags such as -Os, note that in some
cases these flags actually seem to increase size, so experiment before
deciding.

Of course using small C libraries such as uClibc and dietlibc can also help.

If you have any queries, mail me and I'll see if I can help.