The 11/2" is sufficient unless you plan on relocating it a great distance, as there are distance limits from the vent based on pipe size (8' for 2" and 6' for 11/2"). The pipe should not have a drop that exceeds it's diameter before reaching a vented line is the basic premise.
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"The trouble with people idiot-proofing things, is the resulting evolution of the idiot." Me
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