What sort of a plate is the Sunda plate?

The convergence of the Indo-Australian plate with the Sunda plate is an oceanic-continental convergence. The Sunda plate carries some major land masses: much of SE Asia including half of Vietnam, and Borneo (which is itself three times the size of Great Britain).

The islands of Sumatra and Java are part of what is known as the Malay Archipelago, though they belong to Indonesia, as do the Lesser Sundas to the south. They are part of the original land mass of the Sunda Plate and were once connected before sea level rise separated them. They have numerous volcanoes caused by the subducting Indo-Australian plate. Arguably, these volcanoes are gradually making them larger, which also applies to the Lesser Sundas to the south. In 1815 Tambora, on the island of Sumbawa, was the site of the largest volcanic eruption in recorded history. An even larger eruption occurred at Lake Toba on Sumatra, some 75,000 years ago.

Well, in short, it's mainly continental, (oceanic-continental convergence).

But in regards to the "type of plate" that the Sunda plate is, is minor plate or tertiary plate. Meaning it's not very big etc. Because 1 tectonic plate can have both oceanic or continental crust. Tectonic plate and crust are slightly different concepts. So, The Sunda plate is mainly comprised of continental crust. There might be a bit of oceanic crust, but less.

Map of oceanic crust on earth: https://www.ngdc.noaa.gov/mgg/ocean_age/data/2008/ngdc-generated_images/whole_world/2008_age_of_oceans_plates.jpg