We need a POD to make them big enough to clash with the Europeans. How about a POD at the end of the Han dynasty? Anyways, you need to pick it and then we can speculate
The Europeans. They had naval superiority.
Obviously they aren't marching on Nanjing; this instead represents European attempts to break into the Chinese world system.
Chinese sources were always impressed by European vessels, most notably their ability to sail close to the wind. Chinese ships didn't carry as much artillery either.
Now, this could easily have changed. But let's not assume it would.
The Confucian idiots destroyed the fleet and all blueprints to the ships.They also didn't let civilians build much sailing ships--the result was that by the 16th century,a lot of ship building techniques were forgotten.They had naval superiority by the numbers but weren't the Chinese actually ahead in ship building tech at least in the early part of the 16th century? Didn't Zheng He command a fleet with ships that would have made Columbus caravel's look like rowboats?
The British and French had no meaningful presence in the China Seas in the 16th century.British, French and Spanish in the 16th century
You would probably need a Chinese incursion into the Indian ocean or something and they would end up fighting against the Portuguese or the Dutch and they would probably win because European blue water navy was probably better than chinese blue water navy of this periodThis would take into account a much more colonial minded China than iotl but if in the 16th century the interests of China and European powers clashed how would such a war proceed?
China against a coalition of British, French and Spanish in the 16th century.