You can just set the resolution which shown up in the system settings. There are also tools like SwitchResX
which could change the resolution to individuals. But not every monitor supports every resolution.
You can change the display size in browser, the Dock size, the Icon and font size for files on Desktop etc. But you can't change the size of the Menubar and Menu entries, without changing the resolution.
Maybe one Day Mac OS will get a resolution indipentend GUI, so you can change the size to what you want.
Most programs have the ability to change font sizes in their preferences. Browsers also have plugins that will increase the font size. For Firefox, I use "Nosquint". Don't know if it's available for Safari but I'm sure there's something like it. In mail, go to Mail > Preferences > Font & Colors tab > adjust the size to your liking.
As mentioned above, you can hold down CNTL and scroll up on the mouse or trackpad to zoom the entire screen. (CNTL and scroll down to unzoom).