Question about resolution and image dimensions

Hi,
I have a question i'd like some input on:

After working on my RAW file in ACR and Photoshop, I generally save my finished imge as a PSD or TIFF file.
My question is: What resolution and image dimensions should I save this file as?

I generally save it at 300 dpi and at a size of 4"x 6". I know the resolution is fine but I'm concerned with the size - is 4x6 too small in case I later want to make a larger print? I know I can make the dimensions larger by resampling later but isn't that undesirable?

I don't really see the need to save huge (60-70MB) files of every shot in order to preserve the large dimensions. Most of them I may never actually print. What should I be doing? How do others save their finished files?

thanks for any help,

czaharop