Now I don't claim to be an expert in ships, if anything the opposite in that I hope someone else can correct me or add more if they know, but larger sea journeys required more than simple a ship being a certain size. There's a reason a cog didn't cross the Atlantic (ignoring the island hopping of Vinland), despite some of the larger ones regularly being over 50 meters long. It was about a number of rather minor structural improvements in carracks that made them more able to handle the sort of necessities of ocean going vessels.I don’t think you understand what an ocean going ship is or what a river barge is, for that matter. And since those primitive sails are still used today with not many changes, it may be you are shaky on that topic too.
The first ship to circumnavigate the planet was an 85-ton carrack about 20m/70ft long. That’s not a high technical hurdle to clear for any nation with experience trading overseas or fishing on the open ocean, providing they have the motivation to attemp it. But that motivation is a high political hurdle, probably not far off the Apollo missions in our era in ambition, if much less costly in terms of cash. So absent political rivalry or economic necessity, why would the Chinese bother looking far and wide for more wilderness and more barbarians?
Clinker type vessels, like cogs or other longships based on the Vikings, were advantageous in their region because they could bend and flex. This aided in the sort of rollers of the North Atlantic, and clinker vessels were lighter and thus displaced less water. This meant they could travel up river, and go faster than carvel ships of the same size and with the same sail rigging. However the way they were built gave them less structural stability, lending to an general size and cargo limit. These structural limitations also limited the amount of sails that could be used without tearing the ship apart when those sails were used for tacking, as clinker ships simply didn't have the rigidity to support them.
Carvel ships were structurally stronger, allowing not only ships several magnitudes larger to be built, but also for 20 m long vessels like carracks to have much greater sailing rigs. This allowed them to utilize wind power far more efficiently than clinker vessels, which typically relied upon some degree of oar for thrust. Carvel vessels also had the internal strength to support a centerboard and deep keel, which is important in allowing a vessel to sail against or across the wind/currents.
So it was more than simply building a 20 m long vessel, which people did long before the Age of Discovery, nor having the motivation to send ships elsewhere. They needed to develop the technical and engineering aspects of ships like the carrack or caravel that made them better able to handle oceanic voyages.
Now whether the Chinese had some idea of these characteristics in their ships, I have no idea. However getting a Chinese Emperor to declare to build a ship to cross an ocean requires more than simply building one big enough. The Portugese needed to learn the Volta do mar, to learn of ways ships could travel against water currents and winds through indirect routes. They needed Henry the Navigator to decide to try to bypass the trans-Sahara trade routes by oceans and thus develop a caravel purely for exploration using technical aspects from the Mediterranean and Atlantic/North Sea vessels. They needed the lessons learned in the caravel to develop the carrack, and so forth. It was a general process where lessons or problems were handles one at a time.
I'd say the best bet for China is indeed going north along Siberia before crossing the Bering Straight. It involves the shortest distances and requires overcoming the least amount of technical challenges.