I would personally take 10 mg if I had no tolerance, but in the interest of harm reduction, I will say 5 mg. This should get you high and at 10 mg you risk getting nauseous, and since methadone lasts about 24 hours, you don't want that happening. DO NOT DRINK alcohol on it or take benzos (Xanax, Klonopin, Valium, etc.). Wait 2-3 hours before dosing more, methadone takes a while to kick in.
Also, DO NOT SNORT methadone. I've done it. It burns like hell, and won't really increase the high, it just kicks in a bit faster.