what flavor of photoshop are you using?
The easiest trick is, rather than using "save-as", to use "save for web". There you can play with the jpeg compression and see how large the file is in a much simpler and more dynamic way. Additionally, you can also adjust the size of the final image in this step (though I usually do it ahead of time).
It will give you a fairly accurate information about the final file size.
FWIW, i'm a web design geek and live in "save for web" (shift-command-option-S on a mac).