First off make sure you've had a reasonable amount to eat for the day. Then pick a time like 7 or 8 pm and tell yourself you won't have anything else.
For me personally this worked(and still works) quite well. It's a real change, because I was always having sandwiches, chips, popcorn, etc... But after a week or so, I started to really be comfortable with it. In fact, it became quite liberating. But still to this day, if decide to have something other than water or tea, I want to eat everything in the house. Is I was obviously hungry, I would try and have an apple or an orange.
Now of course, this works best if you're not subjected to latenight food everytime you turn around. ie. roomates, spouse, or friends all eating lots of tasty stuff around you. And somedays it's just not going to work because you have to eat late for whatever reason.
When I'm finished my supper, I think to myself, is there anything else I want? If so, I try and have it. ie small desert. Then after I have my extra treat, I don't have anything else. I think I'm also getting better sleep since I quit eating late at night.
Now of course, everyone is different and I don't get as strong/specific cravings as my wife. So it makes it a bit harder on her.
The smaller size is just like the large size, sooner or later there is no more left.