The partition size for the Red Hat depends on how many apps you intend to run on it, but as you already have the XP working for you, about 2 gb for the Linux should be sufficient. However, most Red Hat versions will not see your Windows data partitions by default and if you wish to use those you'll need to configure it to do that. Come back for advice on that when you're ready. If however you want to keep the data the Red Hat uses separately with it, you'll need to make it's partition big enough for that.
Thee's also the question of the file system that you're using with the XP, NTFS or FAT 32. I don't know if Linux can see NTFS, someone else may be able to advise on that.
Assuming that's not a problem, just run the Red Hat installation and choose your new second partitiion for it and it should install without problems and will provide the Boot Menu at bootup automatically.