Hello,
i just wanted to jailbreak my iPhone 3G 16Gb with PwnageTool 2.2.5 on my iMac G3. I changed the root partition size from 5xx MB to 1024 MB, but in the log while its builed the custom firmware it shows a message like:

resizing root filesystem to 523.

Its normal?

I had the same concern myself. I probably ran the pwnage tool with custom root partition sizes about 4 separate times, each one showing a number similar to yours in the log file, before I decided to try add it to my iphone anyway. My root partition turned out to be what I specified, so I just learned to ignore that log entry. Try it and see.