Forget about going to Burma on your own bike, it aint going to happen as long as their present government stays in power (and probably a while after that, too).
The same goes for Vietnam, though some individuals have made it, but still very, very unlikely, the same as winning in a lottery I think. Bike size does not matter.
China I think is possible, but not that simple. You´ll need to organise it via a tour operator in China, and it will take a long time, probably months, and cost you a 4- or 5- digit number of dollars.
I went from Europe to Australia last winter, first planned to go that way, but thought they can keep their China, because its so freaking expensive. An interesting country no doubt, but I still dont regret not going there - there are plenty of interesting countries that I had not seen before, and that I could do for a small fraction of the price.
I think it would´ve boasted my trip budget by nearly 50%, if I had included China in it. Would´ve run out of cash well before Australia because of that.
Also keep in mind, that getting into China usually means just that - getting into one of its provinces. Crossing the entire length of it could be a different thing altogether, as you would officially need permits for each province, need to pay your guide every day you spend there, etc., so the costs just pile up.
There have been reports of doing China "unofficially", without any guides etc., and some of them may be true, but I dont think you can count on succeeding in that. You´re much more likely to be rejected at the border.
Getting into China (the cheap/unofficial way) on a Chinese-registered bike has worked, check out www gt-rider dot com forums, theres a travel report about this.
About China Im only speaking of what Ive read or heard, but Im sure other people who have actually done it, can tell you more.