You tried. You get an "A" for participation today.
Ack! The dreaded Participation Trophy.
Nobody's really answered WingChunChick's question, y'all. Lots of "Just don't be there" type stuff, which I find funny, but I bet she doesn't.
She asked, "What things do you do to prepare for people who are larger and/or better grapplers?"
In a nutshell, my answer is "practice grappling."
Expanded upon, find a grappling school and go train for a few months (it does take about that long to get a feel for it). But, since your underlying art is WingChun, even though your going to take class and pay attention and try to learn and practice what they are teaching you, when not on their mat, consider what you are learning and how best to integrate that into your own personal system to be able to escape the grappler, evade and get back up and force him/her to have to come inside on your WingChun whirlwind of "now-warned and very motivated" strikes.
If really attacked, and not trying to play the game of grappling, e.g. rules based matches, tournaments, MMA bouts, etc... your goal should NOT be "Fight to Win," but "Fight to Survive." Winning in grappling involves trying to get the other person to submit to you, which, maybe I'm reading into this, but doesn't sound like your motivation. It's to Not be caught in something which could hurt you, or worse. Thus, fight to survive.
Of course,.... you may end up coming over to the Dark Side and hanging out with us weird-o converted strikers now grapplers too, and that's OK.