[CentOS] Using ramdisks on CentOS

I'm running the 2.6 xen kernel on CentOS 5.3. My goal is to use
larger-than-normal ramdisks. I can see that the rd module is statically
compiled into this kernel, so I tried passing the option ramdisk_size=262144
during boot, but dmesg shows that it gets reset to 16384K. I can also see
that this is the size stipulated in the kernel config.

Do I have to rebuild the kernel to make this work? If that's the case, can I
rebuild it with the new size and still use an initrd? If the answer is no,
how do I go about making sure that the drivers I need are compiled into the
kernel? I know the most basic steps to building a kernel, but I'm not
entirely confident I know what I'm doing here. I've looked for howto's but
haven't found anything covering this case yet. Any help is appreciated.
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