CS5's automatic Metadata variables do not reflect the InDesign document's properties, only that of imported images. (That said: I shudder at the thought, but what do they say about a placed InDesign document!?)
But it sure is possible to script this, although the changes will not be “live” — each time you change the dimensions or user, you have to re-run the script.
First create two variables, both of type “Custom text”. Name one “Dimensions” and the other “User”, and insert these whereever you want. Then run this Javascript to set (and re-set) the contents:
v = app.activeDocument.textVariables.item(“Dimensions”);
if (v.isValid && v.variableType == VariableTypes.CUSTOM_TEXT_TYPE)
v.variableOptions.contents = Math.round(100*app.activeDocument.documentPreferences.pageWidth)/100+” x “+Math.round(100*app.activeDocument.documentPreferences.pageHeight)/100;
else
alert ('Variable “Dimensions” doesn't exist or is of the wrong type');
v = app.activeDocument.textVariables.item(“User”);
if (v.isValid && v.variableType == VariableTypes.CUSTOM_TEXT_TYPE)
v.variableOptions.contents = app.userName;
else
alert ('Variable “User” doesn't exist or is of the wrong type');