Alternate to the API docs, with help. This is for PC:
title - string - This text will appear in the title bar of your WebDialog
scrollable - boolean - This is a NOP. You get scrollbars, regardless. Workaround: <body scroll=no>.
key - string - If this is nil or an empty string, your dialog's size/location is stored in "HKEY_CURRENT_USER/Software/Google/SketchUp7/WebDialog_". You should definitely include here the name of your Ruby, which will be concatenated:
"HKEY_CURRENT_USER/Software/Google/SketchUp7/WebDialog_MyRuby"
width, height, left, top - integers - size/location, in pixels, of the first appearance of your WebDialog. All subsequent appearances will be taken from the registry entry.
resizeable - boolean - if false, your user cannot resize the dialog. Defaults to true, if omitted.
Edited to include help from Thomas Thomassen and Jim Foltz. Thanks guys!