What is the largest hurricane possible?
I am going to take an educated guess here because it is not possible(AFAIK) to accurately predict with any known skill what several decades into the future would be like.
Given that premise the largest hurricane in the future could be the size of the tropical extent of the Pacific Ocean or the Atlantic Ocean(wherever that begins and ends). Here I am only considering the Northern(or Southern) tropical extent of the Pacific or Atlantic Ocean because as we know a tropical cyclone cannot cross the equator as explained in this in depth answer Impossible or improbable? Hurricane crossing the equator. The thought process behind this idea is that hurricanes(tropical cyclones) dissipate on coming contact with land. Hence the maximum area of the largest cyclone in the future would  have to be the ocean body maximum tropical extent(typically sea surface temperature (SST) greater than 27 degrees centigrade). Just in case if people are wondering why just the tropical extent and why not more than that ? It is because once you enter into mid latitude regions frontal processes could kick in(cold core cyclones-as an example -Can a tropical cyclone form in mid latitude oceanic waters?)
So if the tropical extent of the biggest oceans increases in the future one can imagine a very large possibly synoptic scale tropical cyclone.
Here I am excluding the North Indian Ocean basin because it does not have the surface area to compete with the North Pacific or North Atlantic
Secondly from this popular science article -How strong can a Hurricane get? and this one Are Category 6 Hurricanes coming soon ?

By the end of the 21st century, human-caused global warming will likely increase hurricane intensity, on average, by 2 to 11 percent, according to a review by NOAA's Geophysical Fluid Dynamics Laboratory, revised on Aug. 30, 2017.

followed by

But physics dictates there must be a limit. Based on ocean and atmospheric conditions on Earth nowadays, the estimated maximum potential for hurricanes is about 190 mph (305 km/h), according to a 1998 calculation by Kerry Emanuel, a climatologist at the Massachusetts Institute of Technology.
This upper limit is not absolute, however. It can change due to changes in climate. Scientists predict that as global warming continues, the maximum potential hurricane intensity will go up. They disagree, however, on what the increase will be.

So theoretically a Category 6 Hurricane could have speeds of more than 200 mph.But scientists disagree on these calculations and i am going to leave it at that.