If you're talking about a flat dish for serving food:
* Bowl: This is the most common antonym, especially for holding soup or cereal.
* Cup: This works for serving liquids, like tea or coffee.
* Tureen: This is a large, covered bowl for serving soup or stew.
If you're talking about a flat, thin piece of material:
* Chunk: This implies something solid and three-dimensional.
* Block: This suggests something large and rectangular.
* Sheet: This is a more general antonym for something flat and large.
If you're talking about a geological plate:
* Continent: This is a large landmass.
* Oceanic basin: This is a large depression in the Earth's surface filled with water.
To give you the most accurate antonym, please tell me what you're using the word "plate" to describe.