in order to prevent the black bars from showing at the top and bottom of your screen, you need to choose a resolution which matches the width to height ratio of your default resolution. The MacBooks usually default to a 16:10 ratio, so you could use a resolution of 1440x900 (not helpful as you stated), 1152x720 or 1024x640 as alternative to your default resolution of 1680x1050.
If the gain in text size still is not sufficient, then you can search the App Store for a screen magnifier programme. I think I once saw one there, but don't remember its name.
Hope this helps,
I have SwitchResX not registered. (starting from 1680 x 1050) I have created a 1440 x 900 res in "Custom Resolutions" menu by clicking on "+" on the left-bottom corner.
After this i've choosen 1440 x 900 res from the "Display Resolutions" Menu
If you clik on the "eye" you can see this
In this way i have eliminated the black raws on the top and on the ground of the screen.
Besides this i use Mozilla Firefox because it remember the "cmd +" command for the pages that i see frequently; so if i close Firefox and than i re-open it, it will display the same page with te size increased.