I have no martial arts training.
When I am facing someone who is 1 inch taller and 50 pounds heaver than me and has been doing martial arts for over 12 years how should I react.
I cannot run away and the situation usually involved grappling. I am reasonably fit. I do not want to escalate the situation.
Are there some basic techniques I can practice?
This is a tricky one to answer without knowing more about your specific situation. If I have misconstrued your question then please add more detail so others don't also get the same impression.
Are you a willing participant, or an unwilling one?
If you are unwilling, to what degree? Is it just a casual nuisance (someone comes home from training and wants to use you as a reluctant training partner), or this is a domestic abuse situation?.
If this is domestic abuse, get the authorities involved now - there is no such thing as "too soon".
If you are simply a reluctant participant in someone else's games, then learning to hit back is not necessarily the right answer either. There is no magic switch you can hit to just stop someone - not without a lot of practice and training. You "don't want to escalate", so this leaves one option - use your words. If you can't have a serious conversation with this person to stop this behaviour, then refer back to my previous answer - get outside help now.
But just to be really clear (in case I wasn't): we cannot and will not teach you or coach you out of a personal situation that involves violence. In situations like this you need to reach out and get assistance from those around you, including friends, family, law enforcement and social agencies.
If someone is more willing to fight than you, more athletic than you, and better at fighting than you, you will usually lose. Try not to be in that situation.
Let me add some "psychological" aspect to the great answers above. This is of course by no means based on any scientific study, just my own experience and reasoning.
If you've never been in a fight, in a tense situation you're usually and obviously stressed, afraid and diffident. This is perfectly natural and obvious - usually people feel this way when they face an unknown situation, especially when it's potentially dangerous. This intimidation is perceived by the aggressor and usually it makes them more confident and eager to maintain the pressure and engage.
If on the other hand you had some sparring and a fight is not a completely new situation for you, it makes you calmer. Of course, you still think "ok, this guy is twice as big as me, and probably is more experienced in a fight", but at least what is potentially going to happen won't be something completely new to you. This doesn't seem much, but the sole fact that you're not as intimidated as in the first case makes also the aggressor less encouraged. Which helps avoiding the fight you probably were going to loose. And let me stress this: you don't have to be a macho, look the other guy in the eye and say "c'mon, show me what you've got". Just the fact that you're not completely unfamiliar with fighting and are more capable of remaining calm does help much.
