(I'm not a physicist & I hope this question isn't too basic/lame)

The question says it all & assuming:

• No atmosphere
• No Earth rotation
• Not a single object in the ocean(s)
• Ignore all other disturbances (earthquakes, etc.)
• The ocean is 'perfectly' still

If (due to-me-unknown reasons) the level wouldn't rise, how big an object had to be, so the sea/ocean level rise would be measurable?